I actually thought about doing this with Booth. I know for certain that I had a great interview with my interviewer. We even sent multiple emails back and forth after (he wrote something to the effect of I really enjoyed meeting you, please keep in touch, let me know if you want help with course selections, etc.). I actually wrote him a post-mortem, saying that I didn't get in, but thanks again for your help. I didn't ask for criticism.
First off, the interview is only one component of your application. And, in most places, the interviewer is not privy to the rest of your application. I conduct alumni interviews for JHU and sometimes I'm surprised by who gets in and who doesn't. Last year, the girl I thought was best wound up on the waitlist. The point is that your interviewer may not have any useful feedback for you, especially if it's a student or an alum.
Second, the schools won't give out much info on why you didn't get in. I don't think it can hurt to ask them, especially if it's phrased in the context of "I'm planning on re-applying next year, how can I make my application stronger"? I actually think this is your best shot. But, I doubt you'll get much other than "we had a very strong applicant pool this year and we can't admit everyone".
The other thing is.... You're not rejected yet!!! You had interviews at these places. Maybe you'll get in.
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