If you've converted you Knight to Chaos, then I don't mind you using them in friendly games. However, I hope you don't mind me using Storm Ravens with my Dark Angels as part of my army so I can put my Deathwing in them. Or Centurions, I would love to use all my SM models with the army I've been playing for 16 years. Something about calling my DA successor chapter generic iron and green Space Marines rubs me the wrong way. Also I hope you don't mind me taking Wraithknighs in my Dark Eldar army without using allies to take the Eldar HQ and troop options.
And I know that it's not the same (or what you Chaos players want), but Chaos already has a super heavy Knight-like unit. They got it before anyone else did, other than Orks. I do hope that in the future they put out rules and a FW conversion kit for Chaos Knights. They are in the fluff and would probably look really cool. Honestly do you really need Knights? I for one would hate to fight 3 or 4 Heldrakes backed up by a Knight surrounded by cultists. That doesn't sound like a of fun to me, especially not in a casual game.