Just to play devil's advocate with the whole proxy points thing, there are a lot of things that you can contribute to a fight aside from pure damage.
Smashing up shields, menacing cavalry with a spear, nudging enemies to set up a kill, and even just diverting an opponent's focus/blocks are all incredibly useful tactics that help your team win a fight.
Even your very presence alters the way that nearby allies/enemies will play, because they can only assume you will help/attack them.
How exactly do you quantify an individual's contribution to the team's success when there are many radically different ways of helping?
Proxy points are an inelegant solution to this cumbersome problem, but at least they encourage players to fight alongside one another.
(Is my support bias showing yet?)