This is a bit exaggerated. At least from my experience, majority of people in my roulettes play OK and they go smoothly: healers AoE, tanks mit, dps do their thing so the run takes expected amount of time - bellow 15min for dungeons.
There was a post on reddit where somebody did the stats on all the duties on mentor roulette grind and by his estimation 17% of the players played their jobs bad enough to be noticed.
As for what is needed, yesterday I had a run of Smileton. After the first pull I was curious what is happening, since Scholar was not using any healing abilities whatsoever (well, technically, Selene :) ) - they were not needed, I was tanking on WAR. I believe he only cast succor outside combat on me for chip dmg when pulling and some whispering dawn for unavoidable damage.
WAR shenanigans aside, there are not enough mechanics that damage dps that don't stand in bad. There is nothing for healer to do than be moral support mascot in this scenario.
