See my comments on the first response. I don't mean to dispute that the branches have different cultures today or moral is affected by cultural continuity.
It's not so much morale (although that is important) as it is about effectiveness: someone effective at Marine combat won't be effective at Air Force combat, and vice versa, and similarly for the Navy and the Army. They're different jobs, and different people do well in them.
Certainly different jobs require different skills, but do the jobs inherently sort themselves into a few branches which ought to operate with some independence? I don't know the answer, but it doesn't strike me as an obvious yes.
