My department doesn't have a rotation program where students are admitted strictly based on their application packets and then spend time working in different labs to figure out where they want to do their degree. There are pros and cons to that system, but it wouldn't work in my department for a variety of reasons. Instead, we do a mixture of our own recruiting and interviewing potential students who apply to the department, but we only accept students who are matched with a particular lab when they come into the program. Everywhere I have worked has been like this, and honestly, I like it that way.
One major drawback however, is the dance one has to do with the candidates. I typically interview 4 students and rank three. One of the factors that has to go into that ranking, however, is whether I think the student will come if accepted. This is critical because once you extend a letter of acceptance through the grad school, the ball is in the student's court. In an ideal case the student will accept and you can then quickly decline the others on the list. More often than not, however, the accepted student takes some time to decide. During that time I am left stringing the other students along, not wanting to tell them that they are not my first choice while hoping not to lose them right away to other labs. If my first choice declines I need to have a pool to go back to, which is why timing is critical and why the pool can quickly dry up if the accepted student delays too long.
I can try to impress upon the accepted student that a timely response is helpful, but I can't demand a response until the official university deadline. Of course, this deadline corresponds with the deadline of several other schools in the area and if the candidate waits until then to decline, I'm pretty much screwed. In this case I would probably be left with a decision between an applicant who did not get in anywhere else or not taking a student at all. Although it is possible to find students who turn out to be excellent in the lab after a mediocre undergraduate experience, the odds are stacked.
In the mean time, the music plays on...