We traded for Favre because Pennington was essentially wiping the floor with Clemens in camp. We knew Chad wasn't going to be the guy to get us to the next level and Kellen was doing nothing to help his own cause.
The Jets got Favre because they could and felt no matter who won the starting job, was the best chance to win in 08. If Chad won, we know his limitations. If Kellen won his inexperience would have lost out to a healthy Favre.