Of course it's worth something, indeed it's worth very much. It's not about reducing armor to zero, it's about negating damage mitigation. The less armor you've got, the more damage mitigation you'll lose per points of ignored armor.
I know Arena cloth can compete or even beat leather, but it's still in a bracket where each lost point of armor will increase physical damage by a worthwhile amount. All things considered, ignored armor will give a nice damage boost against clothies or leather wearer and a little boost vs mail and plate.