First, thanks, Goldielocks, for saying what I wanted to.
Second, this encapsulates it for me: "I have come to accept MBTI as good for one thing. A way for people to articulate the fact that people can receive and perceive inputs/outputs from the world differently despite seeing the same thing"
I don't know -- or care -- whether the test accurately captures some objective definition of personality. But it absolutely helped me figure out how I thought/processed/spoke relative to other people I worked with.
I test out as INTP -- in HS, I was INFP; after law school, the F changed to a T and has stayed that way forever. Most particularly, I am an off-the-charts "N," which in this test says that I care what something means more than the details of what it is. I am also a strong "P," which again in the test terminology means that I don't do too well with too much structure and planning and in fact tend to start to get the heebie-jeebies if things are too planned out.
I am also a lawyer. So you can see how my particular personality traits -- extremely good at the big picture, can frequently see answers others can't, not so awesome at the details of how to get there -- would not necessarily mesh well with other lawyers. Like, say, the bosses who expected me to walk them through every. minor. step. of my thought process and research before they could understand the answer.
I knew those differences existed; I mean, come on, I'm smart, I could figure out that I didn't click with various partners, but I just couldn't figure out why, what I was doing wrong. When we did the MBTI, it allowed me to put a name on it, to understand why -- oooooh, he's a strong S, he builds his way to the answer brick by brick, so if I want him to understand me, I need to re-frame how I present things. And -- most critically -- I could then extrapolate from my experience with one specific S to the *other* strong Ss in the office -- before I fucked myself over by defaulting to my preferred style.*
In short, I don't know if "S" or "N" or whatever has any relevance in the field of modern psychology. And I really don't give a fuck one way or the other; all I know is that it sure as shit helped me in the world of work, because it gave me a way to identify points of conflict in advance and adjust my work style to something that my coworkers/bosses could grasp more easily. The N/S difference is the clearest one to me, because I am extremely strong on the N (30 of 30 in the version my office did), which means I am very strongly anchored in my way of thinking about things, and so the differences tend to be more stark. But the same holds true to some degree with the other indicators -- they have all been very useful to help me figure out how others around me work (e.g., strong J = ok, need to overcome my loathing of tight schedules and come up with a good one so he's comfortable I'm on track).
*The testing also confirmed that most lawyers -- and law firms -- are not N/Ps, but the firm I ended up staying with was disproportionately in that category. Which also seemed to explain why I had always felt I "fit" there better than any other place I worked.