The dps increase for the tanks from tanking 30 mobs that do zero damage for all of about 3 seconds is approximately zero. The DPS increase for the warlocks, who can pump out another 2-3 Chaos Bolts with all cooldowns (which would each crit for about 3-4 million), Bloodlust and trinkets active, is ridiculously massive; the same goes for all classes that gain procs or resources if they debuff or damage a large amount of adds.