That's kind of the point with Passage of Arms, Stack Mechanic Damage where everyone is running towards that Big Stack Marker Arrow can be mitigated by 15% and even better if party members would've died without that mitigation if the damage is high enough. Pretty decent for AOE too if you or someone screws up and won't get out of the AOE fields in time.
It's also pretty useful as a last resort Tank Buster Mitigation if your other Mitigations are on CD and not enough Oath for Sheltron. Your Block is 100%, you can Clemency afterwards if you want to cheese it further. PLD is pretty low risk tank job, cheesing fights with Mitigation and Clemency was fun for a while, then it got boring.
You also look badass with Passage of Arms too, Wings, Glitter and Pretty Glows.