This is quoted from grayracer513 from TT.
"Floating rotors do not dissipate heat any better, and in fact maybe worse, being that they don't have a direct conductive path to the hub. What floating does is it prevents, or helps prevent, the rotor from loosing its flatness when it gets hot by not binding it to a rigid center. It also allows a rotor that may actually be deformed by heat to move around on its center so that it tracks straight through the caliper in spite of slight distortions."