Easy enough in concept, sure. But if you are actually working on the quest when the content is new, there are guaranteed to be other groups farming the PHs. Not to mention, there was no guarantee that you'd get a drop when you came back 20 min later, so you might have to do your swarm over and over till you got the drop (assuming you found the mobs un-camped in the first place).
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-Bumamgar
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