i also tried what you did and here is the result:
my best armor's defense is 2067 / 24.1% = 55 damage from lvl 18 boarfaced imp
my backup armor defense is 1881 / 22.4 % = 57 damage from lvl 18 boarfaced imp
no armor at all defense is 0 / 0% = 73 damage from lvl 18 boarfaced imp
so in conclusion the armor works well and not decorative, the problem is the % is at minimal that you wouldn't notice the difference, you will just feel the changes when your def reaches at 1k... correct me if im wrong...
I hope this helps you... lots of luck...