Why don't people like to pick/use Tenacity over Determination?

[Rasikko]

I'd go for it, if the defense boost was alright.

[Warskull]

Yeah, when I said meteors I meant to say comets. It sucks that you can't use any abilities while "holding" the comet because if you pop invuln too early it doesn't do much good and if you wait too long you can't use it at all. I have enough materia that I can redo my gear several times over so I think I'll give tenacity a try. Thanks for the info.

For the comets do not stand in the final spot. Stand by it near the boss. Once it hits the final spot it glows, you actually have about 2 seconds to pick it up. So you don't have to worry about timing it. You see the final spot, pop hallowed then rampart, then step in it. You get almost 10s of invuln then an addition 10 seconds of 20% damage reduction.

Plus this lets your team focus all DPS on the other tanks comet at the start, that makes the phase go quite smooth. Tenacity probably helps a bit for the other tanks, but as a PLD there is no better place to use hallowed in the 2nd half of the fight.

[Gr1mwolf]

The big difference is in how damage vs defense is valued, due to enemies having insane health pools relative to players.
Having 5% more defense won’t allow you to survive a single extra hit without dying, which makes it functionally useless (I’m pretty sure this is also true of all the 10% DR cooldowns, which is strange). Say your tank would normally die from eating a mechanic followed by a tank buster; having that extra defense won’t stop that from killing you. If one mechanic takes like 70% of your health, and Tenacity reduces that to 67%, you’ll still die in the same number of hits.
Having a couple extra % of damage though actually will shorten the length of a fight, even if it is only by an insignificant sum.
It also won’t reduce damage taken enough to reduce the number of heals needed to get you back to full health.

The only way to make Tenacity relevant would be to either increase the damage reduction to a massive number, like 3x what it is now. Or make tanks have so much defense in general that keeping them alive is a non-issue, and extra defense just means more damage and less healing from the healers over the course of a fight.