I think it would be better just being split evenly amongst the party. Otherwise we'll get weird situations of people deliberately holding back so that the weaker guys can shots in etc, which to me just seems stupid. Powerlevelling is going to happen regardless - if not with group XP sharing, then with people giving low characters incredible gear, etc. I think a better solution to that would be to eventually make it so encounter critters aren't a viable source of XP anymore, in favour of designated PvE areas/quests/domination battles, where it's a lot easier to control what levels that go on. Big PvE dungeon quests that turn you away if you're over level 10, for example.
Edit: Fleshed out non-combat roles would be nice too. I want medics electrocuting the non-exploded dead back to life. And I know how stupid everyone thought it was, but I would've loved a healing needlegun just to give combat medics a pointedly different role. And Leader builds that provide combat bonuses for other players, not just a horde of mercs. Sorry, totally offtopic.