yeah, that is major annoying. Even with DoT still on mobs they may go yellow, and there are alot of mobs in higher levels you do not WANT to damage at times... unless you like to heal them.
No matter what tho' I will never believe that another group should be able to kill a mob with only the 18% HP left, burned Mega abilities and the like gone. Every group that fights an NM should have to fight ALL of an NM. They should also not get the benefit of our farming for pop items, camping or pulling to a safe location. That kind of thing is why my /blist reads like a who's who of party leaders on my server. Kinda like asking for a paycheck... without taking a job.