I don't but this was the example I was given. So yeah? Its not really survivability when it's putting you in a risky position to begin with? It's not making it so you don't die to that there big raid damage you may have to be soaking. It doesn't make it so that you take less damage overall/get more defencive abilities. It heals you. It is a heal. And one at that that is the main filler for affliction and as blizzard said "lines up well with affliction" destruction warlocks are all about FIRE man... FIRE FUCKING FEL FIRE EVERYWHERE. Why would a destro lock decide "I need to drain this guys life instead of using fire!"? they wouldn't. It's a bad idea. It works for demo... But it definitely doesn't work for destro. And no matter how much you say otherwise. It's just bad... They want unique specs... But throw us this?
Why not be creative and add in new stuff... Because they nerfed what was good and went "ah now we have no ideas how we can make this not be bad... drain life works right? For all three specs? Bit of a mismatch for destruction wouldn't you say? Oh no you're right... Yeah it really fits in with the whole FIRE AND CHAOS theme we got going on there!"
Also healing me is the healers job not mine. When would I ever drain life other than at low HP out of range of a healer (LOL BAD RAIDER ALERT) and not going to cause over healing/mana wasted from the healers? Probably... never. So... great survivability... right there.. uh huh... non filler spell.. very niche usage... Lets also remember that blizzard nerfed ember tap so that destro didnt get out of hand with out stripping healers. Drain life seems like it could easily become a out heal the healers spell for all 3 specs.
TL;DR
Drain life is for affi. Unique specs are not unique if they go back to all sharing the same spells anyway. 1 step forward 10 steps backwards.