I think a damage meter would be a really god addition to the game... As it would let you know how much of the mule work you are actually doing for your team.
For example, if a skirmisher runs in and kills a low health Heavy Knight every game, he will get all the credit, even thought it was his team that actually did most of the work.
Don't get me wrong, the finishing blow is important, and I don't think that it has to affect points in any way (thought it would be cool if it did) but I think it would be a neat addition to the score screen, even if you could only see your own.
I know that lots of other Source games already have a similar mechanic, and damage is already counted for statistics, so I imagine it would not be that hard to add (though then again I have never done anything like this so I can't really speak on that.)
Anyways, that is my suggestion.