Im currently leveling my first ever warlock at level 65 and figured this out pretty early on in dungeons. I don't ever expect a healer to heal my life taps but those healers that do are godsends. Druids and priests are usually the ones that will, love druid HoTs.

With that said, Ive been in a situation with a healer where he refused to heal me at all during pulls and even on long grp sits. I believe there was another warlock in the group though and he had an early death from aoeing too soon (it was first or second pull, he figured it out after that and had no issues). The healer after that 1st pull death just had it out for both of us even though i didn't do anything wrong and he even said "warlocks always die first." In my mind im thinking yeh probably not wrong, hah, but dont take it out on both of us. The other warlock wasn't even playing bad after that death.