Knockdown is often just 1 or 2 free hits, and generally causes your death unless you're a massive tin can. I can understand why some weapons would have knockdown, but the ones that make sense also have crush through, so knockdown isn't necessary.
Frankly I think that crush through should be removed and knockdown should replace it (but still knockdown when overhead is blocked), then blunt weapons that don't cause crush through should have knockdown removed. I personally think this would be a lot more balanced for the game.
A stick having knockdown for example, is stupid. Damn iron staff.