I've tested Elemental Drain with Crushing Shock, and it only procs once, it is not worth using at all, add to that the fact that spell resistance apparently doesn't work.
It does precisely what it says it does, restore Magicka when attacking with Flame, Shock or Frost. For shorter duration fights you'd be correct that it's basically worthless, but for long sustained fights it's quite helpful to have it and Siphon Spirit on a Boss (PvE) with 100% uptime. Also, @ThatHappyCat is correct, PvE NPC's basically have no armor/spell resist as of right now so the 40% spell pen doesn't really effect overall DPS more than a tiny amount. But if you require high DPS for a long period of time and can't afford to have Siphoning Attacks up for long (NB) or can't use Spell Symmetry often due to incoming damage and aren't a Sorc using Dark Exchange regularly the added Magicka replenishment is VERY helpful.Then I check the readout from Combat Log Statistics (and a couple of other addons) and they confirm my own, admittedly unscientific, observations that this skill really doesn't do jack. I really wanted this skill to live up to its description - it should be awesome - but it just isn't.