Maybe this was argued in beta, maybe it's a dumb idea, maybe I'm blind, but AFAIK this hasn't been brought up ever.
My suggestion is simple, add armor to PRR.
Heavy armor adds full armor to PRR,
medium armor adds 2/3rds of armor to PRR
light armor adds 1/3rds of armor to PRR.
no armor adds maybe 1/6th of armor bonus (from for example bracers or robe) to PRR
So if you're wearing a full plate that adds 20 armor class to your AC, you get 20 more PRR.
This way the quality of the armor matters as much as BAB and armor size. A +7 Planeforged Fullpalate would grant more PRR than a lowly masterwork fullplate. A +6 robe adds 1 PRR, allowing wizards some tiny amount of damage reduction.
Thoughts?