It is essentially a 33% (or 43% with adorn) chance to reduce incoming damage an additional 5% to 10% for most hits you'd be taking (hits between 6500 to 10000ish post-mitigation would be about 5% effective DR, anything less and it's a higher effective DR) except from the heavier raid mobs... in which case it's more like an effectively 2% or 3% DR.
And combine that with the ward proc to berserk and you nearly have a consistent 2% to 3% or more DR against raid mobs and 5% to 10% or more DR against the majority of heroics
I fail to see the uselessness. It's basically a scaling DR that is less effective against epic mobs.