Nobody receiving damage is a bad idea.
Say there are 2-3 guys chasing one enemy... If they shoot themselves down, the enemy should benefit from their stupidity... Same as if they all augered into trees or hills. Also, by encouraging that kind of stupidity amongst folks, it's not fair that your teammates can cover the bad guy like a magic bullet absorbant shield of power (TM, patent pending) in case they want to grief you and just disrupt your play.
Bad idea, all around, to do NO damage at all.
In reality, you shoot the wingman down. In this game your lands hit the wingman, but you shoot yourself down. End result: Your bad aim is punished, not your wingman for doing nothing, and the end result is the same: the enemy has 1 less plane chasing his six.