Think it thru, GCD is the key. You can never defeat Global Cool Down. It will always cost the full GCD regardless of how fast you cast that spell. So what if your frost bolt is cast in .6 seconds, you still incurred a full 1 second GCD to cast that frost bolt.
As to the why GCD exists, I believe it is because of the server/client relationship in WoW. Aka if it didn't exist, would be impossible for those two too communicate correctly.