Probably because PLD also has Passage of Arms, Cover, Intervention and Clemency. Balance isn't a vacuum where you can just compare skills individually.
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I does appear that the shield scales off the player and not the warrior. Tested this with a crafter war party. Still another mitigation in warrior's back pocket when unbuffed, or party utility.
After reading what shake off does um I don't know how to feel an ut it like great raid utility but hopefully they don't accidentally use it or use it and they end up dying trying to put a shield on everyone else lol pld has shielding skills without all the draw back like loosing all their buffs and stacks and taking extra dmg
shake it off is decent, if you're worried about the losing buffs part you can use shake it off when either one of raw intuition,vengeance,thrill of battle,inner beast are active and the duration almost runs out,that way you will get 12% shield(and more depending on how many cooldown are active) and won't lose anything significant
it's the fact that inner release still share cooldown with unchained that sucks IMO