I'm a DPS Sorc, recently there have been several times that I have queued for DPS in GF, but when GF pops it lists me as the healer. The first time this happened I thought I forgot to uncheck the healer box in GF so I declined group, went back in to GF unchecked the healer box box and queued again. 25-30 mins later GF pops and i'm listed as the healer again. Some of the "healers" the OP is talking about may have been DPS players that were switched to healer by the bugged GF system. This seems to only happen when I'm queued for GF for a long period of time. I have filled out a bug report but have not received a response from BW (ticket still open).
Oh, good, so it's not just me. I had this happen quite a bit as a DPS Merc. I just went ahead and healed the FPs after telling the group what happened. We did fine and no one died - granted, I'm talking Hammer Station and Cademimu here - so nothing requiring heroic measures, especially since both tank and I were over-level and well-geared. But annoying nonetheless.
As to Healers dpsing a bit, I recently spec'd to Bodyguard and, yeah, I'll throw out an Unload or a missle or 2 while fighting trash mobs just to help speed things along, but during boss fights I'm waaay too busy watching health, for the group and
myself to do anything but heal