I've only had to tell a healer that they're awful once, and that was on my Paladin Tank.
We were back in Cataclysm, and we had what was probably the worst Resto Druid I've seen in a long while. We were actually stuck on Heroic Yor'sahj for the longest time because this guy literally couldn't do a grand total of 3 things: Keep Lifebloom up on me, don't Tranquility during the purple ooze phase, and listen to basic instructions for healing cooldowns.
He finally got the message and swapped over to his Hunter when I linked him the healing charts, showing my Paladin outhealing him with naught but Seal of Insight.
I had a druid that wanted to heal Firelands (during t13 mind you...it was a very casual guild) that was doing literally 4k HPS on Rhyolith.
He was in mostly feral quest greens and didn't have a resto set.
It wasn't a very fun night.