I am not, I'm well aware of that defensive buffs are multiplicative, so that multiple instances of Damage Reduction suffer diminishing returns.
So stacking CD's is not as potent as it would seem on paper, but that's why having multiple tools available can be strong since PLD and DRK can both just hop into Defensive Stance basically whenever (It costs a GCD) for a free Rampart in addition to any other CD they wish to use along side it.
Shield Oath + Rampart = 36% damage reduction on a 90s CD. Every 90s.
Shield Oath + Sentinel = 52% damage reduction.
With of course, Shelltron to toss on top of these.
Same thing for DRK. Grit + Shadow Wall = 44% damage reduction. 120s CD
DRK also has the Grit + Rampart for 36%. DR 90s CD
Both can have TBN's on top for 20% more EHP.
If it's Magical then DRK has the Grit + Dark Mind for the 44% DR (With TBN on top) with a mere 60s CD!
DRK has the bonus effect of not even needing a GCD to swap back out of Grit.
Meanwhile WAR, relies upon getting healed after swapping to Defiance for it to give a bonus. Also spending Gauge to activate IB's 20% DR which the other tanks get instantly upon entering defensive stance. Even with the 45% ideal uptime, that's still 55% less uptime than both other Tanks gain for their Tank stances from the second that they swap to the second that they leave. With the only advantage being that you can sit in Defiance and use a IB right before you swap to Deliverance (But there's a 10s CD between swapping stances on WAR so outside of Unchained it's more punishing if you need to swap to get that bonus 20% mitigation for a single hit, such as a TB)
In extended periods of damage, WAR in Defiance starts to balance things out, because it then gains access to its plethora of self-healing mitigation which is its way of balancing against its inferior damage reduction mitigation. But then also note that DRK itself has self healing, especially in Grit where Souleater will heal for 100% of the damage it deals (Compared to Storm's Path's 50%) in addition to having DA'd Abyssal Drain and Sole Survivor CD (On an unrelated side note: Anyone else want FSH to have a skill called Sole Survivor for pun related reasons?)
WAR lacks somewhat in on demand damage reduction relying mostly on 120s CD stacking or taking pretty notable DPS penalties in order to gain IB bonuses. But they end up averaging out when comparing over time mitigation when their self healing comes into play in Defiance. Though the issue is that Defiance (Well, defensive stances as a whole) are not meta. While PLD and DRK gain access to their on demand mitigation in DPS stance as well as have less DPS loss for a quick swap to Tank Stance for a free 0 CD 20% DR boost to empower otherwise mediocre short CD's they can access through Role Actions.