On the validity of MBTI, the main use I've found is to use the eight functions as part of my vocabulary. It's a pretty handy approximation to be able to say to someone in conversation "wow, Stephen Fry has a really developed Fe" and have them know what you mean. There an approximation is all you need, and in exchange you get neat symbols to hang entire categories of ideas onto.
On the flip side, it's all too easy to move symbols around in your head without investigating whether the model you've built corresponds to reality...
On the flip side, it's all too easy to move symbols around in your head without investigating whether the model you've built corresponds to reality...