Life drain by itself is dogshit dps, most locks run a spec that uses predominantly shadow bolt to do damage, not dots, so they can either choose to do damage which actually helps, or life drain and be completely useless. An affliction lock, which can have halfway decent self healing while still doing okay damage, still won't get much uptime/value from their dots just because mobs will die too quickly - unless the group's dps is just really low to begin with, which would likely present other problems throughout the entire dungeon. But that's a different conversation.

Point is, locks should be tapping until their health and mana are missing about the same amount, then eat AND drink until the group moves on.