Thoerycraft: Tanking with Redoubt, but defense capped?

This:
It becomes a waste of 5 points, but not without merit, this allows you to get 3/3 Shield Spec which increases your shield absorb by 30%.
So my question is how useful is it be for Tankadins who stay 440 Def?
Is it useful to stay just under the Def Cap for the Proc on trash? Could perhaps swop out shields for it to proc on trash.
Redoubt (Rank 5)
Instant
Increases your chance to block attacks with your shield by 30% after being the victim of a critical strike. Lasts 10 sec or 5 blocks.
It becomes a waste of 5 points, but not without merit, this allows you to get 3/3 Shield Spec which increases your shield absorb by 30%.
So my question is how useful is it be for Tankadins who stay 440 Def?
Is it useful to stay just under the Def Cap for the Proc on trash? Could perhaps swop out shields for it to proc on trash.