"How dreadful!" cried Lord Henry. "I can stand brute force, but brute reason is quite unbearable. There is something unfair about its use. It is hitting below the intellect." ~ Oscar Wilde - The picture of Dorian Gray
It posits a measurement system for defining personality traits, and TBH the concepts behind the functions have given me a useful (IMO) insight into how and why some people are the way they are, so at the very least I don't think it's useless. Comparing it to such abstract organizational imaginary constructions such as Mathematics is something that makes sense to me. I think some facets of the organizational system are superfluous (like the 4-letter types), but that is purely opinion; I personally find the concept of the functions the most interesting.
Yet while I find the 4-letter types superfluous I do find the concept of the temperaments to be mildly relevant to reality, so I think the most superfluous part of the 4-letter type isn't the type itself so much as the perceived "function order" that it resolves into. So if I were to modify the system, I would do away with the 4-letter types and define temperaments in terms of preferential groups of functions.