The point is that MBTI is a descriptor, wich has some common behaviours and attitutes overlayed on it. I think the reliablity of prediction is pretty weak in places, but useful in others...
E types tend to think outloud/get energiesed by conversation.... etc
When it gets down to specifics and detail I think its pretty poor... and of limited use (but still better than nothing)
I fully agree with defining a book by its cover etc.. but it can be helpful to have some awareness of likihood, it depnds on how the user uses it