In an effort to get this to be a football topic again;
I think mobile guys arguably are the best backup QB's you could ask for. Unpredictable, a completely different style of play to gameplan for, and they make up for their deficiencies as passers (that all backups invariably possess) with that extra threat of taking off. If you need to go .500 when your starter was out, a guy like Vick or Kaep would be the ones who could do it short-term.