I dislike when healers try to DPS instead of watching health bars. Healer DPS is so pitiful, you might as well stand and wait for healing to be done instead.
I'll echo what Kitu said and additionally say that every little bit helps. If they're confident they can get back to healing me when I need it then I'm perfectly fine with that. Then again, as the OP points out - that group would be one I too would get nervous about.
To the OP, that sounds like a case of the cocky-veteran player type. The people who got one toon to 50 and think they know how to play. Perhaps they're bored or trying to relax or cursing at their screen because you aren't tanking to their liking while encounters aren't as quick as they hoped. It happens, you're best bet is to just tough it out and encounter people who you can enjoy playing with and in return they enjoy playing with you and then kill and mame together.