If you know that Favre needs a good running game to succeed, and you don't have a good running game, then why bring him back? Do they have that much faith that they'll be able to turn it around?
A QB who cannot be consistently good without a running game is not helpful. Nobody denies that he can still throw the ball, but I think you hit on it- can he be consistently good enough to win? Maybe I'm wrong, but I don't think he can.