Right now, a healer who wants to provide some power leveling support can remain outside of a party and avoid ever-aggroing or getting hit by a leve mob or claimed mob. And there's not too much now to prevent someone from going AoE madness and killing unclaimed mobs.

Griefing is going to become an issue.

Can you imagine trying to fight your enemy, then get hit with an AoE that someone else is fighting or worse, that someone dragged over to you?