I've got two healers at 80 and have only just starting doing randoms on my now level 29 warlock so until recently I have been a little biased toward healers having the short end of the stick. I've come to realize that this isn't the case after just one Gnomeregan pug. It was the first time I got called out for my self-inflicted damage.
"Thank the warlock for that.", as the healer needs to stop for mana. Of course I am doing everything I can to heal myself and I wasn't really being a drain on the healer's mana so far as I could tell so I simply replied, "you're welcome". A little later the healer stopped to drink again and I thought it would be amusing to poke fun myself and said, "Give the healer a moment, he's drinking for two." Finally, near the last boss someone asked, "Why is the warlock taking so much damage?" I couldn't help myself and blurted, "I hurt myself...", "I get off on it. I know it's wrong, but I can't help myself." The RP realm guy was heartily amused as was the rest of the group and from that point on there wasn't another word on the topic. I've never really gotten into the whole role play aspect of my toons, but it has been something I've had a growing interest in. In this case it was an easy way to diffuse an otherwise awkward PUG situation without going into debating class mechanics. This little taste of role play has wet my appetite for more.
Unlike the middle of a PUG, now it is a good time to talk class mechanics. Fact: Warlocks life tap to regain mana. There really is no other way around it in a group environment, but warlocks also have the ability to heal themselves and take that burden off the healer. I'm speaking from an affliction perspective since that is what I play, but I am pretty sure that is the case for all warlocks. The main problem is that those two things don't happen at the same time, warlocks lose health to regain mana and then start regenerating health with life drain or using a healthstone. By that point the healer in me would see the missing health and fill it up much faster than the warlock can themselves and any healing the warlock would do would be overhealing. So the warlock continues to DPS and life tap while the healer shakes their head in disapproval.
The take away for healers is to give warlocks the opportunity to take care of themselves, or more specifically don't top them off if you are having mana issues. It is a good indication to the warlock that if their healer isn't topping them off that they should focus more on taking care of themselves. There are going to be bad players on both sides of the fence, healers that don't heal warlocks and warlocks that life tap to the point of death and expect heals. I actually felt bad life tapping as a warlock until fairly recently since I've only just started to get the tools to heal myself to make up for it. Now I realize that as a warlock you have to learn to strike a balance between life tapping, DPS and health regeneration depending on how much extra healing is coming your way. Some healers say go nuts and others will outright refuse to heal you if you life tap. Most, including myself when I am healing, are somewhere in the middle.
It is mostly a matter of common sense on the warlock's part, don't life tap unless you know how you are going to get that lost health back; when in doubt drink. If I am low on mana and see a promising life drain target I will dot it up, life tap and then start sucking that life back. One of the first things I learned in Gnomeregan was I can't get health back from mechanical targets, bummer. In between pulls I will life tap all that a healthstone will afford me and make a new one for next time. When I see a target with low health that few other people are attacking I will soul drain it and attempt to proc a mana boost. That is how I get most my mana back soloing, but it requires me or my pet to get the killing blow. Another thing to be aware of are hots ticking on you, if you are at/near full health then use those hots to life tap rather than let them get wasted as overhealing. One thing is for sure it is never the healer's responsibility to manage a warlocks mana, but a little help goes a long way.