Generally, a good healer can make or break a group when it comes to success, especially if something doesn't go to plan. You're pretty powerless in those ohfuck situations if you're dps or tanking imo.
in WoW i felt that this was the case until Cataclysm. When they changed the mechanics, I felt it became much harder to compensate for other people's ineptitude if you were healing than in the previous versions of the game. It is possible that with gear/other patches/people learning how to do encounters, it has gotten easier, but I know I struggled healing when I first hit 85. I have since stopped playing though, so my opinion is probably moot.