armor capping on non block tanks

But in order to add in that additional % damage reduction through an increase is armor added to the already average % consistent between the tank classes, would this pose the risk of forcing armor wow po capping on non block tanks.

Don't get me wrong I love the idea of a static armor capped reduction, but on top of the passive (talents) damage reduction of bears already and the active (cooldown) reduction available to DK's would this model be ungoldy overpowered in a pvp setting? Or for that matter even in hard hitting melee PVE encounters.

Again forgive me if I'm totally off here on my thinking, theorycrafting is no where near my specialty.