I'm butting in again! Discussing the 'science' of MBTI is good. Sometimes we're too caught up in the theory and forget that it's just a theory, a hypothesis that can be reformed at all times.
Even if a person is very well developed he/she might still have some sort of preference. If anything you can still look at his/her four letters. My dad uses good feeling functions to communicate to others and can pretend to be extraverted and all that, but he's an INTJ.
One question regarding to the MBTI theory that I have, (I posted it in another thread btw): If the inferior function is so bad, then what about the other four 'unused' functions?
So is an INFJ or an INFP worse at Se? It's the inferior of an INFJ, but nonexistent in INFP. Si, though, is the third function of INFPs.