Well in the original plan for the justice system it was said that we would be able to kill vendors and quest NPCs, only that they would be instanced to us so as not to inconvenience other players. First part of the justice system is live and...you can't attack or pickpocket vendors or quest NPCs. I'm guessing that that feature ended up being more complex than they at first thought and was dropped...which means I don't expect to see it brought in on an even larger scale.
I wonder if they could just flag certain mobs to despawn as soon as the corpse has no loot.