#1 People of similar attractiveness do tend to gravitate toward each other.

#2 People of all levels of attractiveness are able to measure other people's attractiveness on the same scale with relative accuracy.

Dan Ariely has a very good talk on this over at bigthink.com:

http://bigthink.com/ideas/20752

Given this, I think stratifying dating sites on attractiveness would actually improve successful match rates. Whether you can do this all in one site or need to create stronger logical boundaries is really a matter of implementation.