It's starting to look like tanks will be getting some offensive love. PLD gets real damage combo, WAR gets a DPS stance....lol I still can't believe that but i love it, DRK burns mana for extra damage.
I assume that every job will have an ideal uptime for tank and DPS like WAR does currently. <Assumption> And so it's making sense in my head that you'll actually want to tank swap to maximize DPS. WAR takes MT with Unchained + Vengeance on while other tank uses their damage stance + combos. WAR MT runs out of cooldowns and the other tank takes it back.
I don't know if PLD/DRK will have anything like Unchained + Vengeance to increase their damage while MTing, but I have a feeling that DRKs will have a good reason to stop OTing once they are out of MP.
Generally Tank swaps are just for dealing with forced tank swap mechanics so that those rascally FFXIV players don't just bring a 5th DPS like the dev team didn't intend. I think there could be some viability to tank swapping for solid DPS reasons since mandatory tank swaps are probably going to still be a thing.