You are correct, it is not everyone's sole intention to "butter up" but I ask you this; how often do you think people send thank you notes to policemen, garbage collectors, job interviewers, professors, firemen, airline pilots, SDN moderators, etc.? If you do, then great, but I would imagine most people don't (or I just live in a very cynical world) so again, these committees are professionals and they have seen it all. Just like writing I want to help people as your main line in the personal statement is trite, so is sending a generic thank you letter to your interviewer. I have had this discussion with some of my classmates before and they disagree with me also so this is just my opinion but I believe in it.