I know people have given their pros and cons of keeping Kyle Orton for next year, but I'm not sure they've condensed it into a post like this, so here we go.
Orton came to the Chiefs whose offense had struggled all year to move the ball and gain first downs. After only being with the team for a month and injuring his finger, he put together two 300 yard games in his first two starts. A QB who can consistently throw for 300 yards a game is a good QB who is more than serviceable. He's a QB that can lead a good team to a Super Bowl (yeah I said it).
The Chiefs have won too many games to be in position to pick up a top-of-the-draft QB, and we all know that Pioli isn't going to go crazy trading picks or players to try to pick up the first pick, and anyone who thinks he is or that it would be worth it is really living a pipe dream. Since Orton has shown the ability to move our offense well (and one would think that will only get better the longer he is with the team) he is the best QB option we have at the moment. Sign him to a three year deal or so comparable to Hasselbeck's in Tennessee.
What to do with Cassel and Stanzi
Stanzi can continue to learn under Orton and Zorn and Cassel can either be a really good backup option, or we can trade him. A better option is to keep him on the roster until next year when, inevitably, a starting QB for another team goes down and Cassel's trade value goes up (see Carson Palmer).