My impression is that unranked waitlists, at least in part, favor those students who really WANT to go to that particular school. So if you can write a "first-choice" letter telling them that you really want THEM, they're more likely to really want YOU. However, the unranked thing might also have something to do with preserving the diversity of the class. In other words, if a URM acceptee declines admission, they'll take a URM waitlist-ee to replace that member of their incoming class. I'm not sure if this is necessarily the case, but I've heard a number of people suggest that it is.