Timing is a major key, life tapping while the tank is pulling draws attention away from the task at hand. It also draws away from the healer's mana. My healer gets very frustrated when a Lock takes over his mana pool. To the point where he wanted to take a third rogue instead of a Warlock in a PUG. Life Tap is a cool ability in extended combat situations, but Warlocks can eat or drink at the end of a fight just like any other class.