I only take yellow mobs from people who take mobs from me when I'm changing pets or kiting. Or if they wait until I use Fight to snag mobs from under me repeatedly and laugh about it. I appreciate courtesy just as much as the next person and will give anyone else around me room to operate so long as they give me that same courtesy. The moment that's broken though, I stop being nice and will call up buddies to lock down the area and troll the offending douche bag until they give up and go home.