I believe combat skills are ammo based and not cooldown based due to the weight mechanics. A character that specializes solely in combat abilities can carry up to four weapons but will incur a hefty recharge penalty. I would assume it was decided that players would trade an unlimited usage of powers in exchange for more guns to swap to for specific uses. Where a biotic might then use nova to damage nearby enemies, a combat oriented character simply swaps to a shotgun.
There's also the fact that some combat abilities have incredible damage potential, and always having them on had makes a lot of things trivial.
With all that said, I entirely agree that having ammunition on powers is not something I'm a fan of, and I agree even more when it involves hybrid profiles.
