I am well aware that interviewers are often volunteers rather than faculty members. However, they are still doing the job they signed up to do *for the school*--I think that's my main feeling about it. Whether they get monetary compensation or not, they are doing something for the school, not for me. A polite smile and a sincere thank you (if warranted) at the time of the interview is all that is required.
For example, if an interviewer said she/he was going to recommend that I be admitted, I would certainly thank her at the time of the interview (and be sincere). But, I've also been in interviews (many years ago) where the interviewer didn't even attempt to find out anything about me. Does that require the same response as the first scenario? I don't think so. There's polite and then there's patsy. I'm not going to send out a thank you letter that isn't sincere (unless it's for a wedding gift and then if you don't, people get mad).
Sometimes the most polite thing to do is to say nothing at all. 🙂 If I can't send a thank you note that is actually personal--not just that fake generalized thank you, then I won't send it. "Thank you for taking the time to interview me" doesn't mean much to the person sending it or the person receiving it. At the most, you'll get someone to think "Oh look...a thank you note." Now if that person actually had some impact on me, I might reconsider my position...maybe if she gave me a tip on a good horse....
Will~