Two reasons for the mickey,
1. BFBC2 and BF3 are both multi-threaded and perform better with more cores.
2. The audio (from BFBC2),
Naturally, doing all of the audio processing in software puts the CPU under some extra strain. Since Xbox360/PS3 has fixed hardware and both have multiple cores available to do many things in parallel, you could say this is only an issue for the PC SKU where we may end up having less cores than on console. In Bad Company, all audio processing is performed sequentially on a single hardware thread regardless of platform. For PC this means that a CPU with a higher frequency will help more than one with more cores. But of course, there are other areas of the game that execute in parallel, so having more than 2 cores will help the game in general. This is an area we're constantly looking to improve and the results of our efforts will show up in future titles.
Essentially means that having more than 2 cores allows one to be dedicated to just the audio processing, and I don't see BF3 being any different. This is/was pretty much the biggest gain in that game when moving from a dual core to a quad core.