It's the combination of factors. I really don't trust other school's decisions much. It doesn't make a candidate more delicious because they got in to a particular school. Heck, I just saw some really scary people get into really good schools!
If the most desirable person on your waitlist is holding an acceptance at Harvard, he is no longer the most desirable (since you can't get him!).
If the most desirable person is holding at a school just as good as yours and you have no reason to believe he will come, it will cost you up to two weeks to find out (and there is a good chance he will decline).
If the most desirable person on the list is holding at a "worse" school it might be wise to offer sooner (before he has moved) in order to increase his likelihood of affirmative response.
A person holding nowhere can still be desirable, but one can put off this decision a bit, often.