Would it really be "rushing" Mayer? His main problem has been injuries, but he has done well at every level he has played at. And he has been in the system a lot longer than Campbell.
Campbell has played in AAA, but not very much. And AAA is really not much of a developmental step in many organizations, since it is significantly populated by ex-MLB players on minor league deals.
Mayer probably does not have Campbell's ceiling, but his roster spot is heavily dependant on an injury right now. Campbell, per Soxprospects. is "passable" at shortstop. But that same source has much higher praise for Mayer at the position.
But then this is all only if Story gets hurt, and the Sox will likely keep the Kid Gloves on with him until opening day...