I've played far too many games where I've had mobs stolen from me. I'll be happily fighting a mob with my melee character, and some over-powered mage will come running up behind me and nuke the mob with a one-shot spell. He not only gets the xp for it, but also the loot, and I'm left with nothing but irritation. (Yes, this is a specific example, but other similar situations have happened as well.) If somebody claims a mob, nobody else should be able to come in and kill it off, AND get sp/xp/loot/whatever off of it. First person or group to hit it should get the claim.