I was discussing this topic with a couple friends who also tank FF14 and have tanked in other games as well, and the bottom line is that PUG tanking is the double edged sword of improvement.

On the good side, playing with people you don't know, who have wildly different skill levels, class combos, attitudes, etc simply makes you a better tank. There's honestly no better way to get good tanking chops than PUG tanking. It's just so random and unpredictable, and that builds your skill set.

On the bad side, while you're getting those skills in those PUGs, you have to suffer through all manner of wankers criticizing you, talking nonsense, deliberately making your job more difficult just to have a reason to criticize you, etc. It's the bill you have to pay in exchange for getting the skills. That is unappealing to many people.