Actually, I know mana regen isn't a big deal with us locks. Life tap and we get more.
I don't know your spec, but with my demo lock, I CoA, Corruption, Immolate and then spam Incinerate. Doing a heroic instance and we encounter a few mobs of elites, after I DoT them up, spam Incinerate, redo DoTs, if needed... after the fight, I need to life tap, due to low mana. I sometimes have to life tap multiple times, during boss fights. Yeah, a healer can usually heal me, but having more life isn't for those 1-hit kill fights. It's so I can life tap more, without making such a dent in my life.
Also, correct me if I'm wrong, but I've always thought and even read here that:
hit > SP > haste > spirit > crit
Crit is the last thing locks should aim for.