Unchained. Seriously, why are we not abusing this skill?

[Reynhart]

Every good tank views their tank stance as an additional defensive CD.

It is, but in your case, you're not talkin just about Defiance, but Defiance + Unchained, which means that you're restricted by a cooldown. Besides, what makes WAR great at mitigation is Inner Beast. Without it, it has clearly less mitigation tools than PLD or DRK. So that defensive setup, in this situation, is cheaper for PLD and DRK than for WAR.

I'll make calculations on how PLD and WAR would compare on these 20 seconds :
Let's suppose that both tanks are in DPS stance, with every combo already active (Maim, Eye, Goring Blade, Halone...)

As a WAR, you pop Defiance+Unchained, then do the combo you mentionned : Eye combo (150+190+270) - BB combo (150+200+280)+ HS (150) + Maim (190) for a total of 1580 potency. With buffs, you have 1580*1.2(Maim)*1.1(Eye) = 2085 potency with a 25% eHP increase thanks to Defiance.

As a PLD, you pop Rampart, then do (at worst) Halone Combo (150+200+260), RA combo (150+200+340)+FB (150) + RB (230) for a total of 1680 potency. BUT, you also have a tick of Goring Blade and Sword Oath for approximately each GCD adding up to 800 potency. I'll remove two ticks of Goring because the GCD is lower, so 700. So 1680+700 = 2380. For the same eHP increase thanks to Rampart. On top of that, PLD will probably have Sheltron available to stack.

So, if the mitigation from your tank stance is enough to survive those 20s, then WAR is actually weaker than PLD on that window (And keep in mind that I didn't count Storm's Eye for the PLD). So, it would be better to let the PLD tank for that time, in SwO with Rampart, and use those abandon stacks for Fell Cleave while staying in Deliverance.