My two cents...if you know you wouldn't attend the waitlist school over where you are already holding an acceptance, why stay on the waitlist? Just to see if they take you, and then decline it the same day? That seems kind of odd to me. If you are accepted at school A, and waitlisted at B with no real intention on going, why just stay on it to see if they decide to admit you in May or June? That's just creating extra work and scrambling, for no real gain.
I can understand waiting to see if you get an interview invite from somewhere (I have a few that I probably wouldn't go to at this point, but I've been complete there for over 5 months and haven't even gotten a hold yet), and then maybe declining it. But waiting to see if you get an acceptance is just doing a disservice to someone who might be waiting on pins and needles on that waitlist. Once again, if you don't agree that's fine, but that's how I see it.