Well, the Bills certainly weren't a bad team in spite of their QB's either. Granted, lousy quarterbacks wasn't the only reason for yet another losing season, but neither Edwards or Fitz would be starting for any other team and Brohm was riding 2nd pine at Green Bay.
Fixing the OL, while absolutely necessary, is no guarantee that Edwards, Fitz or Brohm are going to be any better. I don't think Edwards is ever going to get his confidence back in Buffalo and I don't think Fitz is ever going to get any more accurate than he already is. Brohm is an unknown commodity with potential, but unknown nonetheless.
McNabb wouldn't be the worst move for the Bills, but I would rather see them get a QB on the upside instead of the downside.