MT/OT as an "MT tanks the boss the entire fight while OT handles other mechanics" paradigm is a really old and ineffective way of approaching fights. When considering efficient play, both tanks would be in their DPS stances and you'd swap either to handle mechanics and/or so that you can more aggressively use the CDs on both tanks. This is due to how strong our defensive cooldowns are and how XIV's fights are largely timeline scripted. With the latter especially, there's no reason to maintain the MT/OT model where one tank is always CD-starved while the other is pretty much wasting theirs by not using them, as you can plan everything around the fight timeline. The only thing that remains is that WAR is the best tank to pull with because of Unchained and because there's no cost to swap to Deliverance. This isn't going to change unless WAR gets a penalty attached to stance-dancing, but that was already something they did away with after it was poorly accepted on SB release.