Just as you said, it is not black and white. Everything is situational. I've seen plenty of times where tanks or heals (even those who should know better) were trying to DPS where they shouldn't be, because they're being pressured into DPSing more or because they think they're better than everyone else and can force their partner tank/heal to do everything for them. Or, something goes wrong and because the tank/heal was DPSing rather than doing their job a wipe is caused. And yes, there are times when it is much better to "do nothing" rather than try to swap stances and DPS, especially in PUGs.
In PUGs, its almost always better to er on the side of caution. You don't know who is in your group, you don't know their skill level, you don't know if they're going to do something different than your used to (another really common thing I see in PUGs...people assuming a specific strat is going to be used. Then you see the tethered people going in opposite directions and killing themselves, and throwing a bunch of stacks on the other group...). As I said, go all out with people you know. But PUGs? You're the fool for expecting more than the basics.
The whole Cover thing...if a DPS is getting cleaved, guess who's fault that is (especially considering only one FCoB boss actually cleaves consistently...Imdugud, Bahamut and Kaliya have a cone AoE that's easy to avoid, and Phoenix only has Revalation that AoEs around the tank); getting hit by the boss's attacks, unless you're tanking, means you're in the wrong spot and you need to move. No excuses, stop being bad DPS. Also...what are the healers doing instead of healing in this situation?
Just for grins, your Lodestone is interesting...you sure talk a lot for someone who hasn't cleared FCoB yet and has a very Augmented Paladin showing. I do wonder just how much you really PUG (or even run) endgame content.