I agree with you, Coriolis!
People already make all kinds of assumptions about who you are based on various criteria that aren't all that informative. It's not even a bad thing, after all, people have a justifiable interest in knowing who they are dealing with. So long as the assumptions they make have a better than random chance of being accurate, given the observation they are based on, and so long as the people making assumptions realize those assumptions only have a PROBABILITY of being correct (and, can therefore be wrong), it's useful to be able to extrapolate observations about a person to get some idea of what they might be like and how they might act.
MBTI mostly focuses on the kinds of cognitive processes a person uses the most. As such, it's a great tool so long as it's not taken out of context and used to define everything about someone, even traits that aren't measured by the test. I've certainly found it very helpful in understanding how people around me think.