I always thought of tanks as the player who takes one for the team. In dungeons, the tank is not actually needed to do the encounter, but it makes life easier when they're around. You are at the mercy of your healer, and your dps will attack whatever they want with no consideration for you. Your job is to adapt. You hold aggro. You ignore what you want to attack, and attack what your dps are attacking. A raid is where you're needed. A dungeon... not so much.