I think not allowing Knights to be field by Chaos players was a poor decision. Pretty much every Imperial type has fallen to Chaos at some point, and I doubt Knights are any exception. I'd agree that they don't need their own codex though. Personally I'd be inclined to see them best fit into some sort of AdMech list.
To take an Imperial Knight with chaos: Step 1: Take an Astra Militarum force as your primary detachment. 2 vet squads + 1 command squad Step 2: Bring 1 Imperial Knight Titan as your Imperial Knight allied force Step 3: Ally your Astra Militarum with chaos forces Step 4: Praise our Dark Prince for allowing large amounts of cultists to die in his name, whilst we receive an Imperial Knight Titan to aid us in our battle.