In motorcycles they do float, there is a tiny bit move movement so that they centre, The force keeps them alined but sometimes they do loosen up, we have punched a few to sort them out, but eventually they rattle loose again. it's normally pretty rare. The benefit of 2 Piece (not floating) rotors on a bicycle is for heat dissipation. The light weight aluminium centre is a better heat conductor and will draw heat away from the steel blade surface, thus minimizing brake fade. The lighter than steel quality also allows them to make the shape of the carrier have a greater surface area for further heat reduction.
