As a tank I can honestly say I do not mind Healers dpsing as long as I know they're doing it.
Run in, cast Holy a few times, heal me, we're all good.
What I do not like, and this has happened to me, is I'm running with a goup of Melee dps and you as a whm want to sit back out of my range of vision and cast Stone II on a mob that is not marked.

If we're not doing a mass AoE pull there's nothing but single target dps in the party I'll mark my mobs and keep loose hate on those not focused on so if you want to quietly sit in the background and pick on a non marked mob, well, hope you don't mind getting punched in the face until I get him back.

It's amazing at how many people can't follow 1,2,3 on mobs.