I'm a lvl 20 rogue, and I gotta say, in all my time questing, I have never seen a monster to have his str lowered to the point of helpless/autocrit. So I gotta wonder, with the upcoming Opportunist feat (+3% double strike), is Crippling strike useful at all? Another benefit I suppose is that lower monster str = lower monster to-hit, but I haven't been able to check that.