The functions are the original formulation, and the letters (preferences) being a Keirsey/Bates addition. Usually later practitioners improve a theory but in this case I think a step back, they make it easier to get started but harder to go anywhere. This is why I have to keep hearing extroverts telling me they are introverts, they don't understand the not so subtleties of the functions (they see it as either/or).
Short answer; functions. Preferences are a fart in the wind.
You have this completely backwards. The dichotomies are the solid part of the MBTI; and the faux-Jungian "cognitive functions" are what's stinking up the joint.
And the dichotomies were not a "Keirsey/Bates addition." They came from Myers.
Carl Jung — mystical streak notwithstanding — was a believer in the scientific approach, and Isabel Myers took
Psychological Types and devoted a substantial chunk of her life to putting its typological concepts to the test in a way that Jung never had, and in accordance with the psychometric standards applicable to the
science of personality. Myers adjusted Jung's categories and concepts so that they better fit the data she gathered from thousands of subjects, and by the start of the 1960s (as the leading Big Five psychologists have acknowledged), she had a typology that was respectably tapping into four of the Big Five personality dimensions — long before there really was a Big Five. And twin studies have since shown that
identical twins raised in separate households are substantially more likely to match on those dimensions than genetically unrelated pairs, which is further (strong) confirmation that the MBTI dichotomies correspond to
real, relatively hard-wired underlying dimensions of personality. They're a long way from being simply theoretical — or pseudoscientific — categories with no respectable evidence behind them.
Buuut contrary to what some of the function aficionados would have you believe, the scientifically respectable side of the MBTI is the dichotomy-centric side — and the dichotomies differ greatly from the so-called "cognitive functions" in that regard. The functions — which James Reynierse (in
"The Case Against Type Dynamics") rightly characterizes as a "category mistake" — have barely even been studied, and the reason they've barely been studied is that, unlike the dichotomies, they've never been taken seriously by any significant number of academic psychologists. Going all the way back to 1985, the MBTI Manual described or referred to somewhere in the neighborhood of 1,500 MBTI studies, and as I understand it,
not one of the many study-based correlations reported in the manual were framed in terms of the functions. The third edition of the MBTI Manual was published in 1998 and, as Reynierse notes in that same article, it cited a grand total of
eight studies involving "type dynamics" (i.e., the functions model) — which Reynierse summarizes as "six studies that failed, one with a questionable interpretation, and one where contradictory evidence was offered as support." He then notes: "Type theory's claim that type dynamics is superior to the static model and the straightforward contribution of the individual preferences rests on this ephemeral empirical foundation."
As I'm always pointing out, the modern function descriptions you'll find in Thomson, Berens, Nardi, etc. differ in many ways (large and small) from Jung's original concepts, and appear to be a set of descriptions more or less jerry-rigged to match up reasonably well with the MBTI types they purportedly correspond with. (As one dramatic example, and as described at length in
this post, the description of "Si" you'll find Thomson, Berens, Nardi and Quenk using bears little resemblance to Jung's "introverted sensation" and is instead a description made to match MBTI SJs.)
Buuut Myers was a nobody who didn't even have a psychology degree — not to mention a woman in mid-20th-century America — and I assume that background had at least something to do with the fact that her writings tend to somewhat disingenuously downplay the extent to which her typology differs from Jung. So it's no surprise, in that context, that the introductory chapters of
Gifts Differing, besides introducing the four dichotomies, also include quite a bit of lip service to Jung's conceptions — or, at least, what Myers claimed were Jung's conceptions — of the dominant and auxiliary functions.
And alas, Myers' lip service to the functions created what proved to be a significant
marketing opportunity for a handful of MBTI theorists who've made names for themselves in the last 20 years or so by peddling a more function-centric version of the MBTI. And for better or worse (and I think it's unfortunate), both the CAPT and Myers-Briggs Foundation websites have long reflected the attitude that the MBTI "community" is basically all one big happy family, and — within certain limits — dichotomy-centric theorist/practitioners are free to be dichotomy-centric and function-centric theorist/practitioners are free to be function-centric, and everybody can sell their books and hold their seminars and it's all good.
And one of the sad results of all that is that MBTI forums are well-populated with posters like you who've swallowed the mythical Berens/Nardi version of the MBTI's history hook, line and sinker.
If you're interested, your deprogramming could begin with
this post, which has quite a lot more on the dichotomy-centric nature of the
real MBTI, and which also discusses the bogosity of the Harold Grant function stack — where INTJs are Ni-Te-Fi-Se, and INTPs and INTJs have
no functions in common. FYI, that's a model that's inconsistent with Jung, inconsistent with Myers, has never been endorsed by the official MBTI folks, and maybe most importantly (and unlike the respectable districts of the MBTI), has no substantial body of evidence behind it — and indeed, should probably be considered all but
disproven at this point.
Talking about how an INTP is an "Ne/Si" type — and therefore has "Si" stuff in common with an ESFJ that an ESFP (for example, as an "Ni/Se type") doesn't have in common with an ESFJ — puts you in the same category as someone talking about how the same INTP and ESFJ have some similar personality characteristics because they're both Capricorns.
And that's a pretty damn farty category, if you ask me.