Breaking news: a few people still play Warlock and many of them still drive healers crazy. Bravely posting on a Level 1 Alt, Suepah of Wildhammer posed the question, "Why do healers care if I life tap to 10% or so?" She says that even if she taps down when the tank is pulling, she uses a bandage and stays out of the way of the mobs.
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.