For dungeons, once sufficiently geared, you'd only need tank stance for large pulls. The individual bosses and other limited target fights such as primal fights, savage, etc. you'll only use your tank stance for initial enmity. The rest of the fight will be spent in your dps stance.
DRK and PLD can use their stance as an extra defensive cooldown if needed, though they rarely are.

You generally have two reasons people will be against this. Enmity and damage reduction.
Enmity is easy, everyone plays with it. This is why you pull in tank stance but swap out shortly after. Between diversion, lucid dreaming, and tactician/refresh, everyone has ways to reducing and lowering their enmity. Plus with Shirk, any tank swaps will only boost the MT's enmity further. It shouldn't be an issue.
Damage reduction is also easy. Nothing can one-shot you without suitable warning. Fights are heavily scripted, everyone will know when the tank busters happen. When a cooldown will be needed. Very rarely would you need a cooldown in addition to a tank stance. Ideally you'd never need it. The most dangerous thing to a tank in the game is big pulls in dungeons, and that's why I mentioned they're generally done in tank stance.

Of course, both of these assume everyone knows how to play decently enough. If someone makes a mistake you can't readily expect anything to work out very smoothly, no matter what role is at fault.

On a final thought, tanks that do everything in tank stance do a lot less than the 20% some people think they do. The slowdown to the kills are rather noticeable. I think the only worse thing to kill speed is a non-dpsing healer lol.
The benefits of using your dps stance is a lot more than just attacks dealing more damage. The abilities you can use in the stance, along with things being unable to kill you if they're dead first (or just phase changing), just make the entire fight easier.