It only makes a difference if you have max armor + an elemental chest piece.
I didn't know it could stack with chest armour thanks for the tip. Haven't used warlock much just started one when ttk came out and haven't played much with it.
It's best on Sunsinger. Radiant Skin + Ram + Solar Chest piece. Try it. You can survive golden gun with that.
I did warpriest hard fight max toughness and three blows later in the bubble I died