Years ago, when I was just a wee student worker in an admission office far, far away, an admission officer showed me an article in the Wall Street Journal about a new practice that was growing in popularity. Interestingly, when I looked the article up this morning, I found that it was written by none other than Dan Golden, the author of The Price of Admission. Anyway, that's not the point...
The article was about the practice that is sometimes referred to as "yield protection". Schools that use this "strategy" often waitlist top applicants under the assumption that those students are using the school as a safety and will opt to go elsewhere when they make their final decision. This has become common enough that outside consulting groups offer to give admission officers some sort of predictive rating for each applicant (don't ask me the details of that process...I don't know much beyond the fact that it exists).
Here and there, I'm asked about this practice and whether it's used at UVa. We do not practice yield protection at all. The applicant pool here is so broad that it'd be hard to compile a profile for a student who wouldn't enroll. I think the practice might be more popular at smaller schools.