Very few teams do. The only teams that survive with a backup QB are teams that didn't have a bad ass at QB to begin with. If the Raiders or Jags lose their top QB, it's no big deal. If Brees, Brady, or Manning goes down, those teams are probably fucked. That's the thing with backups. They're backups for a reason. They're usually career backups. Very rarely does a backup get a shot and become a star.