“i don’t see how Perry staying in helps Romney at this point. Is his 3-5% in SC really going to make that much of a difference? I doubt it.”
He may do better than that in SC. And those few percent do help - they certainly gave Romney the win in Iowa (i.e., had Michelle dropped out prior, Santorum would have won).
BTW, you should probably come to grips with the idea that Cain is not running. That train has already left the station.