It also doubles the work of whoever is writing the system at the time. Alignment is by no means perfect, but it's simple. It works cohesively and coherently with the rest of the system, and any changes to it or removal of it must be considered carefully before a game starts, because it will have far reaching effects into the rest of the system. Besides, the main idea behind D&D has always been to either play the heroes or the villains; alignment is really just a way of saying which and what type of either you'll be playing.