I think it is a big problem many know next to nothing about function theory.
Agreed.
[rant]
The 4 dichotomies irritate the heck out of me, because they are not only incredibly approximative, but they also completely betray the spirit of MBTI/Jungian typology. Only function theory can accurately help someone determine what type they are most likely to be.
Me, for example: I am *constantly* getting typed as an INFP by tests and such. And indeed, if you look at how so-called Perceivers are supposed to be determined, I fall squarely into that category. And yet I am most definitely an INFJ! I may be laid back and spontaneous and always procrastinating and unable to make snap decisions and so on, but I'm still an INFJ.
But then, of course, INFJs DO have a Perceiving function as their Dominant function...
Only by learning about the functions did I realise that, DUH, I am very clearly DomNi and AuxFe and TerTi! And thus, an INFJ.
[/rant]
As to why I never seem to manage to type other people: it's usually because I know them too well and not well enough at the same time. Typing near-strangers is easy, because I only need to look at the few clues laid out in front of me and find the most likely pattern to link them all. Obviously, though, the risk of mis-typing them is extremely high, precisely because the number of clues is so reduced. But typing people I know well is almost impossible for me, because for every detail that would go one way, I can think of another that would go the other way.