So many times you shoot at an "undamaged" plane flying in a dogfight, u spend 5 minutes struggling, you finally shoot him down and...You got an "assist" !
Many other simulations use this system, and it's working very well (obviously sometimes u got some who try to "shoot the burning plane", but thats really rare,and anyway noone want to be called a victory stealer in a multiplayer game...)
HTC current system is based on mathematic dmg formulas, but most of the time it grants the kill to someone who isnt even fighting the enemy plane anymore, sometimes on RTB...!)
To be honest, if an enemy plane has no visible (or not enough) damages and can keep flying, or even fighting(!), then the "kill" isnt earned, its earned when the enemy plane is down.