chainstay mounts under the chain stays means you can run any brake with a small drivetrain (such as 25/9, a very popular combo) and not worry about the chain rubbing on the brake. I have a frame with lowered chainstays (welded a little lower on the BB shell), low-profile Fly brake, and low-profile mounts, so I can run a 27/10 ratio without any problem. some say that chainstay brakes work better, but I think it has more to do with the dialed-ness of the brake setup you have.