This. A 20% chance of adding ~25 damage (the equivalent of +5 damage) still leaves a S&B character too far behind a TWF character who can achieve the same AC.
It's a nice perk, sure, but the situations where people pull out a shield are going to remain exactly the same: only if you need the blocking DR on an intimitank during specific battles. And, of course, this added damage wouldn't make a difference in that situation.