This part of the issue.
If you are a FA like say a Dustin Keller, and you look at the Bills who cut their 1st string QB, and you know they are starting a rookie most likely in 2013 and they lose Andy Levitre, are you going to want to sign with Buffalo?
When Mario signed last year, there was a perception that the Bills were a good team at 5-2, hit with bad injuries, and were a team that could win 10 games and make some noise BEFORE last season. I believe that perception helped us land Mario Williams last year.
Now the Bills have no QB, a 1st year unknown coach. Where is the benefit for vets to come here?
Build through the draft. Nail the picks. That is and always will be, the answer.