I agree: It's discriminatory to hire one based on MBTI. First, it's not set in stone. Anyone can perceive themselves to be someone they aren't, and if they've taken the test enough times they can "guess" at the answers. Second, no one is compartmentalized. We're all unique, even within our traits. We're not die-hard one group or another, and we can be on the edge of various traits. And heaven help us if we're stressed out and we type incorrectly.
Thirdly...really. It's whoever can do the best job with the best skill level in a particular work environment. If you can't figure out who someone is in an interview, then you need additional help. Certainly more help than the MBTI can give you.