If we make the playoffs, it will mean most likely Cassel would have won us 5 games in a row. Would you really want to make that switch to a guy who has played a total of 14 quarters this season? Just roll with what got you there. In a win or go home scenario, you don't have have time to wear off the rust.
I think the Bulls did this in 99. Went with Rob Johnson instead of the hot hand in Flutie and they looked flat and lost. But then it worked the other way in the early 90s. I guess it would be a judgement call.