End drain does actually work, just not that great. Mobs do require end, its just that their requirements are lower than ours, so you have to keep them very drained to get the best effect. It is most noticeably useful when fighting boss mobs that have annoying powers. End drain has helped my storm/elec defender take out more than one boss who otherwise wiped the floor with me. That being said, I would not complain if the end drain got boosted on most of the attacks.