Some years they may take a lot, some years they may take very few or none.
They use the waiting list as a cushion because predicting which accepted students will matriculate is not an exact science. With people applying to more schools on average, they have more choices usually, which means schools will want a larger wait list as a safety net in case large numbers of their accepted students decide to go elsewhere. Just like you said.
I'm sure that schools take into account a student's numbers when they think about admitting 'em or not. Like a T4 will probably assume that even if they give a top student a full ride, that he or she is still probably not going to matriculate there. They probably look at the person's personal statement and see if he/she just uses a generic one (where you just change the name of the school on the PS depending on which school you're sending it to) that doesn't mention anything specific about their school in particular. They'd definitely reject anyone they are sure won't attend, no matter what their scores are. Thats why admissions advice people say never to let a safety know that its a safety.
Yield is important, especially with the wait lists. I think that if you're on one, and you call them up every week and express your desire to attend once you're on the waiting list, that they're more likely to pick you from it. Thats why they usually even ask you wether you want to be on the list or not, they don't want to take people off of the list (accept them) and have them not attend, because that makes their yield even worse.