As I am currently raiding with both a warlock character and a resto druid character, I can certainly understand where most of the arguments on both sides are coming from. Some thoughts:

- A smart and courteous warlock would never tap as a tank pulls or during a simple pull unless absolutely necessary to prevent deaths. There is a right time to tap and a lot of wrong times to tap.

- For warlocks (and other casters) mana used = damage done (for the most part). So to some degree you'd want a warlock (assuming he's geared ok for that situation) to burn through their mana as fast as possible because that means they are putting out more damage faster, and mobs die faster, resulting in faster moving through instances and less time to wipe.

- "Mana tapping" healers can be VERY efficient overall. I can watch a warlock tap from 0% mana to 100% mana, throw 2 dots on him and heal him up to full. That takes 4-5% of my mana and about 3 seconds of my casting to full up 100% of his. And now with 100% mana he can put out another 100,000+ of damage, all because I took 4% of my mana and 3 seconds of my time to heal him. I don't know of anyone else who can effectively contribute 100,000+ damage with 4% of their mana pool and 3 seconds of their time.

- Communication is the key. The warlock and the healer simply need to have a quick chat before it starts or as it starts, so that they can talk about what will happen. Affliction locks usually don't want to be healed after they tap because they have a ton of life-taking abilities which they use to deal damage. A healer might want to make sure the warlock knows when it is appropriate to tap. Etc. Communicate and don't stress out about it.