Because if I don't overclock the CPU, I'll hear a bunch of "Why didn't you o/c?!?!, you know that GPU is being held back by your stock processor!!!!!"
Safer to O/C the CPU to lessen the possibility of a CPU bottleneck in a GPU benchmark suite.
I'm running a Core i7 860 at a pretty rock solid 3.4GHz, HT and Turbo enabled, which I think is probably adequate. Sure, higher the better, but I want a stable system and I haven't had the greatest of luck o/c'ing CPUs.
As for the overclocking benchmarks mentioned above, I may do a few, but don't count on it being any kind of extensive. As it is, I have over 10 titles to bench in a system configured once for Nvidia, and once again for ATI (all else the same save the graphics card and hard drive) at two resolutions.