I usually don't cast it, but there are times you almost have to.
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THe Elementals with Hydromancer Ves-whatever in the Steamvaults can be fully drained in 10-11 seconds, and their melee is pathetic. A Warlock can take both of them out of the fight permanently (Banish) and without her adds, she'd noobsauce.
This is one of them. Mennu the Betrayer in Slave Pens is another. Without his mana, all he has is melee and totems. No more crazy-powerful lightning.
I can't remember many others.. Lots of bosses tend to have so much mana that you can't possibly OOM them even by the time they're dead. It's nice for caster mobs, since their melee is weak, but usually you can kill them fast enough that it's pointless to drain them.
If you have no health, almost no mana, your HS, pots and bandages are on CD, and your target is under control and has mana, the spell instantly becomes your best friend.. until you notice you have drain life and life tap.. :D