You did some googling to come up with questions, I wrote out a reply, and then you deleted your questions. Sigh.
Oh well, have your answers anyway:
the type indicator results of test takers resolve to a monomial, normal trend?
What exactly are you trying to imply by "resolve to a monomial, normal trend"? I know what each of those words mean. However, the most charitable view of this comment I can make, is that it is at best obfuscative. Give up trying to look smart, and tell me what it is that you are are actually trying to say.
Also, where are you getting this assertion from? (Yes, I did Google it.) You didn't just make it up, did you?
Shouldn't it be bimodal if the types are really all that different? What about the fact that it's common for people to have a totally different personality upon retaking the test shortly after? What's the purpose of the test as an indicator of anything valuable if the type indicators as defined by the test are so malleable?
OK, now these are intelligent questions. As an aside, there are two basic types of question: challenging, whereby the questioner intends to test or undermine the questioned person's authority or knowledge; and informational, whereby the questioner intends to acquire new information from the questioned person. No prizes for guessing which category yours are in!
No. The "MBTI types" are a code for eight dominant functions (Si, Se, Ni, Ne, Ti, Te, Fi, Fe), each followed by one of two auxiliary functions (Si-Te/Si-Fe, Se-Ti/Se-Fi, Ni-Fe/Ni-Te, Ne-Fi/Ne-Ti, Ti-Se/Ti-Ne, Fi-Se/Fi-Ne). The tertiary function is the other of T/F or S/N from the auxiliary function, with the opposite i/e alignment to the auxiliary function (and the same i/e alignment as the dominant function). The inferior function is the other of T/F or S/N from the dominant function, with the opposite i/e alignment from the dominant function (and the same i/e alignment as the auxiliary function.
Thus the types, in ideal form, are Si Te Fi Ne (ISTJ); Si Fe Ti Ne (ISFJ); Ni Fe Ti Se (INFJ); Ni Te Fi Se (INTJ); Ti Se Ni Fe (ISTP); Fi Se Ni Te (ISFP); Fi Ne Si Te (INFP); Ti Ne Si Fe (INTP); Se Ti Fe Ni (ESTP); Se Fi Te Ni (ESFP); Ne Fi Te Si (ENFP); Ne Ti Fe Si (ENTP); Te Si Ne Fi (ESTJ); Fe Si Ne Ti (ESFJ); Fe Ni Se Ti (ENFJ); Te Ni Se Fi (ENTJ).
As we know, humans rarely appear in the wild in "ideal form". The way MBTI works is to attempt to quantify an individual's typical use of each of the eight functions, by asking questions that lean towards strong indication for (or against) one of the eight. Here's a useful analogy as to how that works. Suppose one were to build a fence. The eight functions approach it more-or-less according to this analogy:
Ne - I want to design the fence.
Ni - Why do they want to do this and what is the deal with fences anyway? Is this necessary?
Se - I want to decorate the fence and make sure that it looks stylish and appealing.
Si - I’ll take care of looking at the instructions and making sure that we follow the established guidelines.
Te - Is doing this cost effective? Will it be useful?
Ti - I want to analyze the structure and placement of the fence.
Fe - How will it affect the neighborhood, and what will the neighbors think?
Fi - I want it to be my own special fence that I can share with others over time.
So the idea behind a personality test, is that by examining the person's preferences as to how they actually approach a problem, like fence-building, we can (in theory!) infer their function order. If they answer 50% of the questions in the manner of a Te person, 25% like an Ni person, 25% other, we can suggest to them that they may be Te-Ni (and -Se-Fi), which MBTI codes as ENTJ.
I don't think it should be coded as ENTJ. I think it should be written as Te-Ni-Se-Fi. Not solely my opinion, but just an opinion.
This brings me to your next two questions. It's common for people to test differently in different situations, because MBTI is based on inferences. It's like trying to measure your height by asking you what types of furniture you're taller and shorter than. It's heavily biased situationally. In an employment situation, where the prospective employee is very keen to have the job, it is going to be affected by their desire to conform to their perception of the tester's ideal. In any situation, it will be affected by self-image. As I said earlier, if the naive test-taker holds the belief that one ought to act like a sensory extrovert in order to be successful in life, then they are going to (a) have molded their own personality towards that end; (b) respond according to the idealized self-image even if they actually think differently.
What's the purpose of the test as an indicator of anything valuable if the type indicators as defined by the test are so malleable? What about the fact that the test doesn't have a strong correlation between predicted career success and actual career success?
I repeated your first question there because the answers to these two are again related. MBTI actually is treated as "business astrology". Your prejudice is not baseless. 90% of HR clerks who apply it, have the exact same view of it that you do; the difference is that they believe in (or are willing to pretend to, in order to cut down the sheer numbers of applicants) business astrology.
And again this is the source of the frustrations of people like me, who actually know something about the underlying theory. Jungian function theory isn't "business astrology". It's like those Facebook quizzes about which car you are. You're not a car (even if there is some correlation between your personal identification with a Ferrari vs a Toyota Corolla, and your personality type). This doesn't mean that cars don't exist.
To determine your actual type, you need to think, or be guided through a process of thinking, about the functions. I guessed that you're an INTJ, based on the way you write (explanation-based thought process Ni, external tear-down criticism Te); I'm completely aware and acknowledge that that one or the other may be a dominant function (Te-Ni would be ENTJ), that you may be using different functions to achieve the same end (Te-Si would be ESTJ; Ne-Ti would be ENTP), that you may be using auxiliary and tertiary functions (Ne-Fi-Te would be ENFP), etc etc.
One of the complexities of this field is that humans are very clever, and are quite capable of thinking in a vast variety of ways. Even if those ways aren't completely natural or comfortable, we're still capable of it. (The other four functions, called "shadow functions", are theorized to mainly operate subconsciously.)
If we think in ways characteristic of other types, because we think we ought to, then we can develop the strengths and weaknesses of those types. Jung, and psychologists influenced by him (which is almost all of them, he was one of the greats of the field of psychology, it's like talking about physicists influenced by Newton), are generally the opinion that behaving like types different to our own selves can be a bad idea. It can lead to distress. If we propose to do something (eg practice as an accountant, have a loving relationship, motivate ourselves to exercise), then we should do it as we should do it, not as someone else should do it. Behaving as someone else's type is like wearing someone else's pants; the worse it fits, the more uncomfortable. Do it for long enough, and you may do yourself some harm. Dominant-tertiary looping, for example.
Predicted career success is a guess, and it pretty much amounts to "Would you like to become an accountant? Here's what an accountant does. Does that sit well with you?". Actual career success comes from self-discipline, inherent motivation, goal-driven behaviour, societal influence, self-education, and a lot of other stuff that doesn't boil down to a system of letters.
Jungian psychology is not a source of simple answers. Because MBTI appears, and its practitioners deliberately position it, to give simple answers, the more popular and well-known MBTI causes that criticism to reflect back on Jungian psychology. The purposes of function theory are (1) to understand one's own modes of thinking, in order to develop self-knowledge and prompt self-improvement; (2) to understand others' modes of thinking, in order to interact with them with more positive outcomes.
If it's not a successful predictor, then what use is it?
It's a descriptor, not a predictor. It's not about putting you into a box, it's about showing you that boxes exist, that you have some of these boxes, that you use the stuff in the boxes in various ways, that other people have different boxes and use their stuff in different ways, and that your awareness of which box what stuff comes out of, can help you get it sorted. Your decisions can't be predicted by Jungian function theory.
The abstract ideal type behaves in particular ideal ways under particular ideal conditions. Since you're not an ideal version of the type, and your particular conditions are not ideal, you're going to behave somewhat differently in practice.
If you're aware of your potential behaviour, again, you'll behave differently because of that awareness. And that is the answer to what use it is.
Now, I think I've spent enough time and effort on you today. I don't consider it wasted, because to coherently explain something to someone else, improves one's own understanding. However, I won't be spending any more. Good luck to you, I hope this motivates you to (1) actually find out more about this deep subject instead of dismissing it with condescending arrogance; (2) consider whether you do that in other fields to other people.