A good healer can keep everyone alive no problem. You should not "only watch the tank". If you do that, you are a bad healer. If a DPS gets hit by an AoE, they are bad, but cover their slack, otherwise they will die and it might cost everyone else's lives. An extremely good healer should be throwing damage, and keeping everyone alive at the same time. That is what you should strive for. Don't stick yourself to "just healing the tank".