A lot of tanking comes down to experience, e.g. knowing how to turn mobs away from the party, knowing how to move and keep aggro, avoid AoEs, etc. Which of these skills do you employ in a FATE? Oh, right - none of them. It's not that FATEs make you a bad tank; they simply allow you to level up without ever having to learn how to do anything - and that goes for all classes, not just tanks. Viva la zergfest!