Basically, because of his style of play, you can assume Roethlisberger will miss a couple of games every season because of injuries. So, instead of having two ancient statues as backups, why not have a more mobile quarterback?
No. 2 quarterbacks have limited repetititions in practice so it would make sense to have a backup who can make plays with his legs. Ravens backup quarterback Tyrod Taylor is short and has problems seeing over the pocket, but he can make things happen once a play breaks down.
In that respect, both Batch and Leftwich are vulnerable. Batch was once an elusive QB, but not any longer. The Steelers have had problems on the offensive line for years, but Roethlisberger would pull them out because of his ability to either evade or shake tackles.
With this offensive line and both Batch and Leftwich being immobile I don't see Pittsburgh beating the Ravens unless the Ravens turn the ball over numerous times.