But this is less about fumbles more about the critical. I would only include the 'fumbles' for a save because if there is criticals then there is fumbles. I like the semi-gambling aspect, and so I do want to reward my players for rolling 20's. Not that thats the ONLY reason to reward them though.
But if you were a wizard and my orc rolled a 20 on his save vs charm and you took backlash would you feel like something is going wrong there?