i'm, by far, not an expert, but i have a lot of friends who go to ucsb.
they say rush is not very popular because ucsb is such a social school, so many people find it pointless/needless to rush and spend money on joining a social group.
THAT said..there are an abundance of beautiful, well rounded girls there. Just how many of them go through the rush process? Not sure. So i'd say rec letters are probably not necessary, but they can't hurt, right?
P.s. other people please don't attack the "pointless/needless" comment, im simply stating the mentality of a lot of people at that school and do not agree with it in the least bit!