Tried to do this on my DRG when I just hit 50, gave up for the reason mentioned by JohnMccain, 0.5 sec lost per attack is a big DPS loss and I would say it's closer to 0.7/attack that is lost, meaning that you lose one to two attacks for every 2 combos you do.

Only reason to run combo macro would be when being lazy during low level runs where DPS isn't important and the overgear by level sync makes up for the lost DPS.