Here's how I see it.
1. Things changed the last two weeks. I believe Mayock and even Leroi were correct in saying Manuel would be cut. But nobody expected him to make the strides he made. When you look at things from the perspective of the coaching staff choosing Tyrod over a month ago, EJ really threw a wrench in their plans with his preseason performances.
2. We currently have an exciting but unproven player at QB. He could be injury prone, he could be exposed, or he could break down mentally like Trent Edwards. We just don't know what we have with Tyrod Taylor yet. We need insurance.
3. EJ has shown incredible glimpses of putting it all together the last few weeks. IF he really does put it all together, he becomes VERY valuable to the Bills. Either as the eventual starter or as 1st round pick bait if Tyrod becomes a winner. For all those clamoring to trade him now, I think they're worried that what we saw was a fluke and they're hoping to get a late round pick for it. But if what we saw was a sign of things to come, we'll have the hottest QB on the market next year for QB-needy teams.
4. Since Tyrod is unproven, and EJ is still a bit of a question mark, we know what we have with Cassel. And what we have is good enough to avoid turnovers and move the ball into field goal range. There will be no Geno Smith or Jeff Tuel pick-six meltdowns. That's why he is valuable on this team for $5M which is junk change for a QB in today's NFL.