You have to give him the tools to be successful. If you just throw him unprepared then you will lose games, which is not the goal. If you give him a chance to learn your offense, your personnel and see what NFL defenses are like for a few weeks then you have a much better chance of winning when he does play.
But I agree that it's hard to learn certain things from the bench. I'm one of the only people who advocate giving your developmental QB some snaps during the week. Put in a small package of plays for each week suited to his skill set and let him have a series or two per game, just like you do for every other position as well as in college. I'm honestly surprised that NFL teams don't do this.