I think you're underestimating the mitigating power of Knockdown. Punch, Haymaker, and Knockout Blow can be cycled to keep a single target off its feet most of the time; if you supplement it with Air Superiority like I do on my Inv/SS, it's pretty much all the time. Put a Knockback-to-Knockdown IO in Hand Clap and pair it with Foot Stomp and you're also knocking down crowds of enemies at once. Plus, Dark Melee's doesn't have -ToHit on its AoEs, so by trading the knockdown for -ToHit you're losing your ability to mitigate damage from groups.