There are only so many backups they can use at once. And the more you put in there, the more you are exposing your remaining starters by forcing them to play with lesser players. You only get 45 actives, and WR, TE, RB, DL and DB all use some sort of rotation already. And most of those guys all play special teams as well. What do you do, take out Urlacher, Forte, Cutler, Briggs, Peppers, Tillman, Hester and Knox?