While playing for the Titans from 2006-2010, Vince Young was a solid, not great, starting QB. But really, that's all the Packers need in the guy. When they're up or down 40-0 in the fourth quarter of a late season game, all they'll need is a guy capable of throwing the ball, not a super star. We need to remember that Rodgers will be the one playing when it actually matters. If they win or lose a game, it will never be Young's fault. It will always be someone else's.
Also, let's say Rodgers tears his ACL early in the year. Harrell and Coleman both have solid arms while Young has plenty of experience. In that nightmare scenario, the Packers could just split time between two of them. They could have Young start against a stronger team and Harrell against a weaker one. Now I don't want to imagine a universe where Rodgers is out all year, but I believe that if that was to happen the Packers would still be ok at the quarterback position.