Keirsey's Temperaments try to put 16 personality types into four main groupings. However, the rules governing which type goes into which group are not sufficiently explained. Keirsey entirely focuses on the basic 4 Dichotomies that we see in the MBTI, i.e. Extroverted/Introverted, iNtuitive/Sensing, Thinking/Feeling and Perceiving/Judging, ignoring completely the 8 Cognitive Functions that ordain these dichotomies.
Without a clear rationale, it is simply asserted that out of the four dichotomies, the Sensing/iNtuition divide is the most important for how different types of people connect with each other. Although Sensors and iNtuitors differ in their affinity for processing concrete vs. abstract information, the burden of proof remains for explaining how and why this difference is more significant to our interactions than, say, the analytical vs. affective differences of Thinking/Feeling. Already, the split is entirely arbitrary.
Additionally, Keirsey divides Thinking and Feeling for iNtuitors (NTs and NFs), but divides Perceiving and Judging for Sensors (SPs and SJs). It has been suggested that the T/F (objective vs. subjective) divide is more noticeable in the abstract ideas of iNtuitors, while the P/J difference can readily observed in the concrete actions of Sensors. However, this misses the point that the middle two dichotomies (N/S and T/F) are very different to the outer two dichotomies (E/I and J/P). The middle dichotomies both tell you about the capability a person has with different cognitive functions. For example, a Feeler is going to have a more mature grasp and proficient use of both Introverted Feeling (Fi) and Extroverted Feeling (Fe), while having a relatively lesser aptitude for both kinds of Thinking. The same goes with iNtuition vs. Sensation. For this reason, N/S and T/F intersect well in distinguishing types by their capability with cognitive functions.
In contrast, the outer dichotomies tell you about the positioning and preference of these more and less developed functions. We know, for instance, that those matching in E/I and J/P prefer a similar energy in their interactions with the world. An EP may be more impulsive and improvising in nature, an EJ, more proactive and responsibility-taking. However, these concepts have nothing to do with capability and whether they are best with iNtuitive, Sensing, Feeling or Thinking cognitions. For this reason, prioritising Sensing and Judging (or Perceiving) together creates a grouping that partially looks at capability and partially at preference, a sort of hybrid. In the case of SJs, it just sets out the four types who are capable at and prefer Introverted Sensing (Si). For SPs, Extroverted Sensing (Se).
When Keirsey decided to have NT and NF on one side, but SP and SJ on the other, he created a lopsided, arbitrary distinction. The difference between NF and NT cannot be compared to the difference between SJ and SP as the classifications are looking at different things. Instead, it creates greater confusion; while it is easy to explain how NFs relate to NTs and vice versa, it is impossible to do the same comparing NFs with SPs. Perceiving ENFP will have a completely different relationship with the SPs compared to the Judging ENFJ.
This leaves us with a messy system, ordained by arbitrary and inconsistent rules. Not only that, but the system is severely limited in its use, not allowing comparison between the two kinds of Ns and Ss.
A response might be to say that regardless of the theoretical mess, Keirsey's temperaments should still work and accurately describe similarity. However, even this is not achieved. A major flaw of Keirsey's Temperaments comes from inaccuracy in generalising each of the 4 types in a temperament to having certain traits in common. In the Please Understand Me series on Rationals (NTs), it is asserted that all "Rationals need to gather up abilities. Wherever they are and whatever they do, they strive to perform competently, and usually succeed".
For NFs and NTs, their commonality should only lie in their capability at certain cognitive functions. For instance, all NTs should be capable with iNtuition and Thinking, but less capable in Sensing and Feeling. However, in this video, the commonality is expressed as a fundamental value/preference, e.g. competence. Keirsey doesn't assert that NTs merely have the ability to do the same things. Instead, he asserts that NTs actually aspire to be a certain way. In doing so, the neglect of cognitive functions is made clear. After all, the desire to "gather up abilities" and "perform competently" has all the markings of preferred Extroverted Thinking (Te). His description of NT applies rather well to xNTJs, who combine have Te among their preferences and strengths. However, this description of the NT fails to describe the motivations of xNTPs, who prefer Introverted Thinking (Ti) and focus much more on creating and understanding new theories than any sort of competent, productive output. Oddly enough, Keirsey's description of NTs is perhaps even more indicative of xSTJs who both focus on Te, yet these would be SJs under his system. By generalising Te motivations to all NTs, while neglecting to attach that motivation to Te types that are not NTs, Keirsey misinforms people about the types with inaccurate group descriptions.
One can see a similar issue with Keirsey's Idealists (NFs), where the focus is placed much more on finding a higher cause and changing the world on a crusade. Such a focus is typical of xNFJs that prefer Introverted iNtuition (Ni) and Fe. However, it entirely misses the point of xNFPs with Extroverted iNtuition (Ne) and Fi, who shun lofty causes to focus on individuals and exploring their perspectives in good faith. As such, it is clear that trying to unite these four types under a shared value system would be inaccurate, requiring a deliberate neglect of the cognitive functions that drive these values.
Once again, Artisans (SPs) and Guardians (SJs) face a different issue. Artisans share Se and Guardians share Si, so it is appropriate to look for some shared preferences in these groups. However, Keirsey defines these values through the combination of Sensing and Perceiving/Judging, rather than investigating the cognitive functions themselves. As such some meaning is lost.
For SPs, Keirsey places the focus on spontaneity and freedom. However, this emphasis overlooks the fact that Ne is perhaps even more prone to whimsicality than Se, yet Ne types are relegated to the NFs and NTs. Ne is the function that tries to keep multiple possibilities open, hating to be limited or obligated, yet Keirsey tries to assign this to the SPs. Consequently, Keirsey places too much emphasis on the superficial, free-spirit behaviours we associate with all Perceiving types, rather than going into the unique qualities of Se and how it creates a certain world-view of immediate action and impact on the present moment. This ability to act with immediacy can protect freedom, but it can also be harsh and dominating. ESTPs are notable among the SPs for being rather more combative and ambitious than freedom-loving, e.g. Donald Trump, Alexander the Great and Winston Churchill. Furthermore, in the case of ISxPs, where Ne is especially lacking, but Ni occupies the tertiary position, obligation to a singular purpose, can actually be valued in its tendency to reduce the ambiguity of life.
As explained, Keirsey's temperaments convey a great deal of inaccurate information when trying to generalise each of the four types per temperament. While the values of NTs and NFs are represented entirely by the xNFJs and xNTJs, the SPs and SJs are unified and described in ways that fail to do them justice alongside more rigorous descriptions of the cognitive functions.
Continuing to look at the temperaments as Keirsey describes them, we see that while NTs are presented as the intelligent group, NFs the compassionate group and SPs the exciting, creative (although often less intelligent) group, the SJs are left behind in terms of any special or remarkable quality. Instead, Keirsey stresses tradition, conformity and a certain servility as core to SJ values. In this way, a certain hierarchy exists in Keirsey's system, where the SJs, and to a lesser extent, the SPs, are presented as inferior in their intellect, imagination and independent thought to the NFs and NTs. At the same time, little is done to address this disparity in worth. While NTs solve the mysteries of the universe and NFs inspire the world, the SPs are instead set to painting and fixing cars, while the SJs merely serve people better than themselves.
It is perhaps not surprising then that so few people involved in MBTI identify as SJ types. The SJs, widely claimed to be the most common grouping in the world by MBTI enthusiasts, are also the rarest presence on MBTI forums, or at workshops and meetups. The explanation given by many is that SJs simply aren't interested in personality theories like MBTI. However, if Keirsey's description has any merit, then a highly popular theory that has worked its way into prestigious establishments, such as McKinsey & Company, should not turn these types away.
On the contrary, it seems quite likely that when a group is described in an unflattering manner, where they are portrayed as mindless, servile drones of societal conformity, even those who might actually be SJs may not identify with their temperament as described, and may even feel a social pressure to identify as a 'smarter', 'free-thinking' iNtuitive type instead. In addition, being told that a group is just servile and traditionalist is a sure way to prevent attempts to go further and beginning to understand someone's viewpoints and where they are coming from. From personal experience, these dynamics seem to be at least true in a few cases, where I have seen disparaging remarks made by self-identifying iNtuitors about SJs, saying that they could never form a close bond with someone like that, or telling the rare ISFJ that they probably won't enjoy the meetups that go on (implying that the conversation is too mentally stimulating and unorthodox) and even looking surprised to find out that an NT like myself may actually be dating an SJ. I have even met a few people who seem likely SJs, but cling to their NF/NT typings, seeming disgusted that someone would suggest they are Sensing types. The frequency with which I have experienced these attitudes in others has been quite remarkable to me, and serves as a peculiar example of the prejudices people of an in-group can feel towards outsiders they distrust or feel they cannot relate to.
Such prejudice might be considered an unfortunate circumstance of an otherwise well-intentioned theory, except that Keirsey wanted iNtuitors and Sensors to be separated. In an attempt at social engineering, Keirsey asserted that N types mix best with other Ns, while the S types should be left to other Ss. Two types are seen as compatible as long as they are both N or both S. In this way, two very different types, but both iNtuitive, became seen as ideal pairings, such as ENTP with INFJ. Because of this, it can be argued that Keirsey achieved what he wanted in defining a group of personality types as servile traditionalists, while convincing more intelligent sorts of people that they should not mix with these people if they could help it. However, while a set up may be helpful in the short term to the self-indulgent, it divides people up and creates prejudices and insecurities that didn't exist before. As such, one can make a strong case where the continued use of Keirsey temperaments isn't merely confusingly arbitrary and misinforms people, but that in its divisiveness, it actually can cause real damage to people and their relations with others.
It's probably not MBTI
It is perhaps worth noting that many question whether Keirsey's Temperaments, often measured with his Temperament Sorter (KTS) amounts to simply being a separate theory to MBTI. Technically, the MBTI is what is tested using the actual Myers-Briggs Type Indicator, and that official measure makes little mention of Keirsey's temperaments.
However, there is no denying that many people can and do mix concepts from MBTI proper with other concepts from related and offshoot Jungian typologies. There is something to be said in the MBTI practitioner paying little attention to categories like Keirsey's Temperaments, but simultaneously, most Jungian typologies are trying to understand personality differences, and operate on a similar 16-type base. As such, it makes sense to try to combine different ideas and theories on the subject together, provided they can be shown to be beneficial and increase understanding.
Unfortunately, it seems from the points above that an arbitrary, inaccurate and divisive system like Keirsey's Temperaments won't provide such such a benefit. Instead, if there is need to split 16 types from Jungian typology into four smaller families, then perhaps the clearly-defined groupings from Socionics, such as Quadra and Club, would be more beneficial.