Would PLD be viable if its mitigation was as superior to DRK as its DPS is inferior?

[Syzygian]

Nothing mitigates a penalty. It will always be -20%.

I have to agree with this. You have Maim and Darkside even when out of tanking stance, so the penalty is still 20% (25% for WAR).

Meant to type Darkside, my bad. And no, it does not mitigate the penalty on grit in any way.

Assume Base = everything excluding Sh/SwOath, Grit, Darkside, Bloodweapon, Defiance, & Deliverance

PLD Base = Roughly 7947 potency
PLD SwOath = Base+1440 potency; Roughly 18% damage increase over Base, or 118% of Base as Max.
PLD ShOath = 80% of Base damage, a 20% loss. Or [1-(0.80(ShOath))/1.18(Max))]*100 = 32.2% 
Therefore Shield Oath is a 32.2% DPS loss compared to PLDs maximum damage in SwOath.

DRK Base = 1
Darkside = 1*1.15*1.0375(Bloodweapon); A roughly 19% increase over Base, or 119% of Base as Max.(ignoring BW MP bonus)
Darkside+ Grit = 1*1.15*0.80 = 0.92; An 8% decrease from Base or [1-(0.92(Grit)/1.19(Max))]*100 = 22.7%
Therefore DRK loses 22.7% of Max DPS by turning Grit on, ignoring Bloodweapon TP/MP bonuses.
No Darkside + Grit = 1*0.8 = 0.8; A 20% decrease from Base or [1-(0.80(Grit)/1.19(Max))]*100 = 32.8%
Therefore DRK loses 32.8% of Max DPS if Darkside drops while Grit is up. Actually more if it's down when certain abilities are off CD.

WAR Base = 1
Deliverance = 1*1.05; A 12.5% increase over Base, or 112.5% of Base as Max.(Crit bonus assumed [(.00+.02+.04+.06+.08+.10)/6)1.5] = 7.5% for simplicity)
Defiance = 1*0.75; A 25% decrease from Base, or [1-(0.75(Defiance)/1.125(Max))]*100 = 33.3%
However Unchained = Base 16.6% of the time.
0.3333(Defiance Loss)*.8334(%time without unchained)+[1-(1(Base)/1.125(Max))]*.1666(time with unchained) = 29.6%
Therefore WAR loses roughly 29.6% of Max DPS if Defiance is up, ignoring the higher potency abilities of Deliverance. 

Feel free to correct my math, it's past 3AM for me. The initial poster was on the right track, but holy hell there's so much misinformation going on about tank stances. FoF, Darkside, Berserk, etc do not get to count against tank stance when you have access to them out of tank stance. 1*0.8 & 1000*0.8 are both going to see a 20% loss. Yes this could potentially be offset by ability timing lining up with buffs like Unchained and/or breaks during ability down time. Those variables would be specific to certain fights, and average out in theory. If you would like to simulate 10 minute rotations by fight that might be interesting to see. However, that still will not change the 0.80 modifier on stances.

No. Please read and comprehend first.

What I'm saying is that yes, its -20% from your top DPS out of tank stance regardless, just like the post details. But as you can see, because PLD has no passive that is up 100% of the time and just has cooldowns its like a WAR with Berserk but no Maim and thus suffers a lot more from going into Shield Oath, almost 10% more. (the 32.8% from Darkside falling off doesn't count because no good DRK ever lets that happen in the middle of tanking something).

WAR loses 25% in Defiance but Maim makes up for 20% of that. DRK loses 20% in Grit but Darkside makes up for 15% of that. PLD loses 20% in Shield Oath but... no that's it. It just loses 20% in Shield Oath. Its like the DRK that let Darkside fall off or the WAR that let Maim fall off. Sure it can pop FoF just like a WAR that doesn't have Maim up can pop Berserk.. This is why PLD DPS isn't stacking up. They don't have that passive that the other two have. They have benefits in their OT stance (like the +5% Deliverance or Blood Weapon) and they have FoF in either stance (like WAR has Berserk in either stance and DRK has 6 oGCDs on short recasts in either stance), but they do NOT have a Maim or a Darkside. Its just missing from their kit.

So when you have a tank that is just straight up missing that, it should be easy to see why I phrased it from the direction I did. I realize nothing -actually- mitigates your tank stance.

What I am saying is that WAR and DRK have abilities that make the "reduces damage dealt by x amount" in their tank stance mostly mitigated i.e. only -5% difference from having nothing up at all. PLD doesn't have this. It loses 20% in Shield Oath and has nothing that directly makes up for most of that that is up 100% of the time. It has nothing that brings it up to -5% from having nothing up at all, at the bare minimum. That's all.

FoF is just a DPS cooldown like Berserk or Blood Weapon, having it up in tank stance just means you're using an offensive CD in tank stance just like WAR can use Beserk in Defiance. Darkside and Maim were clearly by design meant to be a full-time buff and (sort of) offset to the tank stance penalty ON TOP of other offensive CDs or oGCD damaging abilities. If PLD had an ability they could keep 100% uptime on, on top of FoF, which is just a cooldown with a set duration and recast as opposed to something you just have up all the time like a stance, then PLD could probably deal competitive DPS.

Essentially I'm saying the exact same thing as all that math, I'm just pinpointing PLD's problem amongst that math.