Any unused cores shut down, while live ones OC themselves and put out higher single thread performance. Plus, when needed you also have a more robust system with 8 real cores, aswell.
So that when future titles like BF3 are out, u have the added capacity to do more.
Nobody (as in gamer/home owner/ end user) cares how fast a cpu is, they only want it to do more, handle more additional loads. Windows7 loves cores.