Battlefield is almost entirely GPU intensive, overclocking CPU wont make much difference at all.
http://www.tomshardware.com/reviews/battlefield-3-graphics-performance,3063-13.html
If your running a 7970/6990/590 you will have a better fps in demanding situations (tonnes of stuff going on like vehicles, bullet, projectiles, construction all at the same time) if your CPU is at 4.5gz rather than just stock for example... it may only be a few fps or it may be 6 fps but in the demanding situations you will get a more stable improvement and better lower fps.
I was running at stock myself even on a 6970 and playing on demanding maps switching between 4.5 ghz and stock on the desktop it was a 7fps difference.
Yes BF3 is very GPU limited, but its also very CPU intensive and while the i5/i7 are good enough at stock to run it if you overclock it does improve stabilty and higher fps in bf3 in demanding situations.
Just because your GPU is running at 99% doesnt mean you CPU cant do better by being faster in certain situations.