That would put certain classes to highly disadvantageous position, and would only cause an issue of "kill stealing" grief. Even if someone with longer cast times would in all earnestly try to participate, they might still not get kills because things are killed while they cast. And some opportunistic runners would not take part in whiddling down the monsters, but just priming in for the killing blow. And healers would active not heal, even if that was needed, if they first need to a sure they get the kills. Or even just "damage done" count.