Mobs die in 2 seconds starting at 60 abyss, how many seconds do you get to focus attack one at a time following the cleric? As for duoing, the dps may still be attacked when mobbed, so what if the cleric draws away 1 mob out of a million?
Another point... If the cleric do by chance miraculously keep ALL mobs to himself, he will be very busy healing himself, so if the DPS do catch fire he/she will burn up rather fast =D, and if the DPS should really follow such instructions as to not deal enough damage to aggro I think the cleric would be better off soloing.