Stop me if you've heard this one. You're taking on some dragon. Some really powerful spell is about to hit you and the rest of the raid. You don't have any cooldowns to use because you had to pop them earlier. The tank who is busy yelling obscenities at said dragon just took a massive fireball to the face and is down in the red. You know for a fact that the next blow is going to be lethal and you have maybe 2 seconds to react.
If you move, you may well have condemned your tank to death. If you stay and heal, you'll end up taking some damage which could be lethal.
Normally, this isn't that big of a deal, but we're in an age where we have so many informative addons that tell us which attacks successfully hit a player and when it happened. What's worse is that this data means the difference between staying and getting cut from a raid (or a guild).
This is a topic of one of the emails that I received from a healing shaman.
So what is a healer to do? Is he wrong for making the decision to take avoidable damage to keep the raid or the tank alive? Is he better off just taking the damage so that he doesn't look bad? Either way, you're damned if you do and you're damned if you don't. Lots of players don't understand that these are some of the decisions that have to be made.