Is there any reason NOT to do this trade?

Absolutely take it. Your major loss would be Earnest Graham because he's doing a really good job as RB for TB. I can't say much for Roy Williams because I have very little faith that he'll produce much more with the QB situation in DET. You get rid of a lousy QB in Anderson and pick up Mr. Dependable Peyton Manning. I know he hasn't been all too impressive yet, but when everything has been said, Peyton Manning is still Peyton Manning. He'll get you yardage and TDs and make up for what you lose with giving up Graham at RB. Go ahead and take the trade.