I have often done this after watching someone fight thier heart out. Saluted em and wished em a good landing. Recently the opponent managed to get home 1/2 wing, at least 2 sector flight to land some good kills (don't remember name).
I will also let an opponent go when damaged if they are no longer a threat and the mission at hand is a priority. The kill for me is less important than the team effort.
And then again, there is the most likely chance that it's just a furball and I don't mind taking them out. I've seen 1/2 winged planes, even planes without wings, take down a friendly in thier death spiral.
Mostly tho I like shooting down my team mates kills after they have taken off thier tail! Must be the Hyde in me