I thought about that a bit... On a desktop machine you don't care much about energy use. Laptops, some people don't care about energy use (at most an hour battery use on the train), some definitely do (on the road all day). So I could see two different designs with say 6 fast cores: One with 6x3.5 GHz, one with 3x3.5 GHz and 3x2.0 GHz. 3x2.0 GHz is not too slow and would take a lot lot less energy. So you would have one processor for maximum desktop power, and one that is reasonably fast for a very, very long time and still has considerable power either for a short time, or when plugged in. (My calculation is that 3.5 GHz takes three times the energy of 2.0 GHz).