I've sent them in most cases. I don't think they help much, since in most cases, the comments are already in place. But I don't think it can hurt to send them, and I do think that in some rare cases, it could hurt not to send them.
Schools want to accept applicants who will want to come to the school. So if there's any questions mark about your interest, a thank you note is good.
It's also just polite and a good habit.