I can't even remember ever asking for feedback from an interview. It's awkward for the employer, especially if they're going through a lot of candidates, and usually if I don't pass an interview I tend to know why. I mean if you're socially intelligent, usually you can tell if you've said something they don't like. Plus I imagine a lot of employers probably have policies against answering that kind of question just for liability reasons, or just to not drag things out.