I think the GP was talking about belief in the usefulness (or reliability or validity) of Myers Briggs. Note that the MBTI doesn't just classify people into 16 categories; it classifies people into discontinuous binary categories along 4 axes, obviously resulting in a total of 16 categories. It's certainly useful to think about peoples' positions along those 4 axes, although maybe there are other axes more useful for describing personalities in some situations. However, I find the idea of absolute binary classification (i.e. you're either an introvert or an extrovert, there are no shades of gray) hard to swallow.

For what it's worth, Wikipedia has some interesting summaries of criticisms of the MTBI's validity and reliability. http://en.wikipedia.org/wiki/Myers-Briggs_Type_Indicator#Val...