Under my suggestion, PRR of 0 would mean that any melee hit insta-kills you. Dividing by zero is bad. PRR of 100 would mean 100 incoming damage causes 100 lost HP, ie no actual reduction, but unlike the present system, stacking more PRR would continue to help.
With your suggestion, someone wearing heavy armor would have basically no incentive at all to equip other gear that adds to PRR. At present the difference between 50 PRR and 100 PRR is bigger than the difference between 150 PRR and 57 million PRR. (Check the formula if you do not believe that).