(1) Armor provides variable damage reduction. That means that every successful attack involves another die roll. This requires discipline, or it will really slow the game down. Every extra: "make an x roll" instruction from the DM is a slow mechanic. The power of the d20 system is its speed and ease, and I think this idea runs counter to that.
Why can't one just roll two dice at once? And only apply the 2nd die when necessary?