You think you've seen bad tanks? Let me tell you about the time I, a Black Mage, tanked nearly the entirety of Amarout.
We get into the dungeon. A GNB, a SCH, a BRD, and my BLM. GNB kinda just... sits there... for 30s before sprinting into the first pull. Without tank stance on. While mashing single target 1-2-3 and nothing else. The mobs spill over the rest of the party, with most deciding to beat my squishy wizard in the face after the first Flare caught a lot of them. SCH plays HP ping pong because the mobs are still spread out over everyone, but pulls through. We tell the tank to turn on tank stance. He silently does, so he CAN understand us and isn't a bot.
Next pull, he mashes 1-2-3 again. I get ALL the mobs except the one he's tunneling. Godlike SCH keeps me so healed and shielded that I rarely get interrupted by the mosh pit around me (first time in AGES I've used Surecast as anything except knockback prevention lol).
We get to the boss. Tank doesn't move for ANYTHING. Not Final Star, not the building collapsing, nothing. At least this time he's actually using cooldowns, which is the only reason he survives the encounter.
We get to the next pull. 1-2-3 and the rest of us ripping aggro everywhere, AGAIN. We tell him to use his AoE next pull, and he does! ...for like half the encounter, and then it's 1-2-3 and mobs beating my face in. SCH and Manaward keeps me alive.
GNB is totally lost in the second boss encounter once the add phase hits. Mobs running between DPS and healer as we keep aggro from one another mitigates the damage. Also the SCH is really good.
GNB does the last set of pulls very sloppily, despite remembering to sometimes AoE. At the last boss, we take the time to describe things like "go on a side platform when the big laser comes out." For the first minute, he actually complies and this looks like a normal dungeon for once. Then, he refuses to move when the boss leaps forward, and dies. Now I'M tanking. I wind up tanking the rest of the fight, because once he's back up the tank never uses Provoke. Then he dies to the laser that he previously handled correctly. The SCH doesn't bother to rez him again, and I tank the rest of the encounter without much difficulty thanks to the SCH.
We criticize the GNB (and praise the SCH) afterwards, because that was a hell of a bad showing. GNB does the shrug emote, and then leaves without a word.
Thanks to the SCH and the BRD (they were using Minne and Troubadour well), it wasn't all that bad an experience all things considered. I even got two commendations out of it (one probably from the tank because my grilling was the least scathing lol).