Honestly I thought the same, but after my first interview which btw was a lot shorter than I expected, I figured I'd send a letter mentioning some positive aspects of the interview and myself. If the interviewers get hundreds of interviewees maybe the resonance of your name or a positive conversation during the interview might get you better placement with them in their memory/application process. If your interview went well, and you have 10 minutes to spare, mail one, or at least email one.