I think that when someone does damage to parts that are vital for continued flight, ie engine, wing, whole tail etc, they should be awarded the kill. I've had both elevators shot off, and manged to rtb and land successfully. Granted, I can in and slid on the big bellybutton square of cement by some hangers, but I still manged to fly back. Same with a wind tip. People can still fly with them missing. Point is, if your plane got to the point where you can't control it anymore, and I've had this happen with just a horizontal stabilizer missing, the person who did that damage should get the kill.
I believe this is fair.
And just so I can get my whine/story in, I remember helping a country man out who had a 205 on his tail. I bnz'ed a few times, (but quickly as I was trying to help him out) and finally got a deflection shot off that took off his whole right wing. Mind you he was flying perfectly before this, following my country man around everywhere. When he hits the ground, Assist on ........