Onerous is not my problem, my...let's call it "uneasyness" is because it gives players a pool of "expandable" HPs. Wanna shoot me? Fine, first shot will just take out my armor, no injury to worry about.

An armor which halves damage and one which provides "armor HP" equal to the character's normal HP are functionally identical, but in the first case every shot hurts, whereas in the second case some damage is on the house.