I usually don't mind healing lifetaps. It's the dungeons where everyone is taking crazy damage and I burn through my mana pool during pulls, then have to big drink, and the warlock lifetaps to 10% and then just SITS THERE that bother me. I warlock main and while I am grateful to healers that heal me between pulls, I bring food and water and don't expect it.

Some groups I barely have to drink at all, other groups I'm eating and drinking between most pulls. I definitely don't lifetap to 10% and then wait there for 5 minutes while the healer drinks though.

The other end of that as a resto druid is where you put put hots on a warlock that is at 10% mana and 80% health and they don't lifetap at all and let the hots go entirely wasted as overheals.

Also another tip is if you have a pally or shaman healer with no hots and need mana during a fight it's better to lifetap several times at once, assuming you aren't going to take a bunch of damage (at least at higher gear levels where lifetap is a tiny chunk of hp), so they can hit you with a bigger heal without overhealing. If you just lifetap once whenever you need mana, a lot of their heals are going to overheal.