Definitely not useless.

A Solar (CR 23) deals 3d6+18 (average 29) with his sword. DR 20/- negates two thirds of it.
A Titan (CR 21) deals 4d6+27 (average 41) with his hammer. DR 20/- negates almost half of it.
A Tarrasque (CR 20) deals 4d8+17 (average 35) with his byte. DR 20/- negates more than half of it.