But even so, increasing the damage penalty on Shield Oath isn't the answer.

I was bored and I parsed myself on a dummy for 6 mins each on my PLD and WAR both in their respective tank stances 100% uptime. I didn't test DRK as I only have the verity weapon. Both have the exact same gear (i325 total) with both having the creation tomestone weapons (i330) respectively and the PLD parsed around 2290 while WAR around 2500. After you factor in slashing debuff and Shield swipe procs they numbers would probably be around 20-40 DPS from each other (probably PLD winning), I'd say that currently even in 100% tank stance they are pretty close to each other. The PLD opener and rotation doesn't change between Shield and Sword Oath, for the WAR, apart from the opener with 3 Inner Beast + Unchained, all other Berserk windows were all done with 4 Inner Beast, plus unchained on every other Berserk window.

Now if we go off these numbers (obviously needs more testing) you can say even in 100% tank stance, both the tank DPS is pretty close. If these numbers say anything, delaying your Berserk and/or your Beast gauge because you are in Defiance is probably a bigger DPS loss than just using it as normal and spamming Inner Beast (needs testing obviously). The only major DPS loss is if you happen to be in the Inner Release window when you're locked into Defiance.

If you start messing with the damage penalty on Shield Oath just to balance out the oGCD stance dancing, PLD is going to end up back into the HW days. While the Western meta has always been tanking in DPS stance, the Japanese PUG meta for the MT is to tank the whole fight in tank stance. While statics is one thing, tanking the whole fight in DPS stance in a Japanese PUG will sooner or later get your blacklisted, but this is another discussion altogether.

But lets get realistic. The chances of PLD stance dancing becoming oGCD is close to zero, at least in the current expansion. I'm getting tired so I'll just leave it here.