I think that thank you letters are good idea. I actually sent a thank you email to my interviewer because he gave me his business card and I felt like we had a genuine conversation and he responded back to the email saying that he enjoyed the interview also and that the school would get back to me with a decision. I feel it's a good idea because helps admissions committees to remember you out of the many qualified applicants as yourself. Don't know how much it helps but it's gotta be worth something.