It all depends on whether you are building a tank that is a dps or a tank that can survive. A tanks job is to be the punching bag that can take the hits while the dps burn down the enemies. All too often these days, tanks want to be the ones in there doing the most damage. I cannot speak as much for the other tanking jobs as I can for PLD (because my partner is PLD), but he has an almost full Tenacity build and can survive waaaayyyy longer than other tanks that I ever try to heal. Even tanks with slightly higher armor levels seem squishy in comparison. And he still has plenty of dps output to properly contribute to the group. Not every job is a full on DPS job in this game. So when used properly, tenacity can be a huge boon to tanks and their survival.