The reason I advocate multiple cores it that you can do all sorts of random stuff in the background and still run your games without lag.
Backround tasks on most PC's wouldn't amount to any FPS hit. Seriously, threads are being swapped out ALL THE TIME to begin with.
8 cores helps when doing 8 heavy tasks in parallel. Due to the nature of PC's [IO waits, memory access, and software design in general], it is VERY hard to use that many resources from a single application. I do not expect to see games, or any other program, scale much beyond 4 cores, and fully expect diminishing returns to kick in.
And yes, more cores does not mean faster processor.