It happens sometimes I chase a plane for a while and a teammate comes, scores a kill and goes away.
Honestly speaking, it doesn't bother me too much. Of course I'd like to be the one who shoots down the enemy plane and gets the score (I agree 100% about rewarded assist kills), but then it's my skills that have to be improved, I can't blame a pilot just because he's better than me at shooting.
"Next time kill will be mine" that's all I think. To me team comes first.
Still, if I see a smoking plane, I usually don't try to finish it off (I think i did once or twice tho, in the rage of battle might happen).
My two cents
Ps. if you all are using headsets, what's better than communicate during dogfight in order to coordinate action and prevent such situations?