I was sympathetic to the "heal only healer" until I really starting doing dungeons as a DPS. I would always DPS when I was a healer, so it was just natural to me because I like being involved. Which brings me to a point:

I was playing Sunken Temple earlier today (as DPS). First time we got wiped because our healer(AST) who was obviously new, did not know the mechanics to the first boss (Doom tiles). So healer goes down, followed by other DPS, then myself, and tank. That's fine--but I noticed the healer would literally just idle during mobs if no one needed healing. Irked me. Our tank apparently wasn't having it, because she disconnected after the wipe and we had to abort.

Second go round, we get another healer (WHM this time), who does THE EXACT SAME THING. No DPS, died on first boss. Thankfully we had good Tank/DPS synergy and I revived her afterwards.

Stop lowering the bar, healers. ;P