Plazek, you have a point that the individual players will do better in the winning team or working together, where you are wrong is that the skill or worth of a player is measured in kills or damage done.
The system doesn't force teamplay on anyone (teamplayer gets the same as lone wolf), the thing is your suggestion would be unfair between "killer" gamestyle and "supportive" gamestyle. In this game you can benefit the team as much by averting damage, creating distractions, etc.
F.e. if I have 8 roofcamping archers shooting at my shield (and throw stuff at them when they get bored) while a crossbowman picks them off one by one, I have significant responsibility for his success, and also for the success of teammates who could have been shot while fighting. But I do no damage (well, except for the 2 throwing axes I hit), let alone kills when I die to the arrow in the side. I'm not claiming this takes skill (RMB ftw), but it's tactics, and those can be employed for the team (take as much responsibility as you can handle, f.e. with a shield you can take a lot of ranged fire), or against it to boost your k:d (run or hide from every fight where you don't have easy win odds and let your teammates die outnumbered). Ofc it's all situational, cav f.e. can benefit the team a lot with this tactic, and in the end there is no formula if you want to have it fair (*).
So, the thing is, regardless how you code it, you'd have a "flawed" formula, which can be exploited all the same. Leechers gonna leech, simple as that.
(*) And it's exactly like that in real life. Noone is really paid by how much his profession provides or how skillfull he is, but by supply-demand and how powerful he is (skill and usefullness figure in, but just count all the useless broken shit that people are making money with). If we had capitalist RPG, shielders would go on "strike" and only shoot crossbows until 2h sign a payraise for them. If we had such a formula though, communism would work perfectly.