A bridge quarterback is not meant to be some big buddy or something. It is really meant to provide the rookie with someone to look up to, to see their work ethic and how they watch film and things of that nature. They are still competing for a job every day. A bridge is not supposed to be your best friend.
There needs to be a veteran in the room. If you are planning on the rookie starting right away, then the vet can be a guy like Matt Moore. If you want the rookie to earn the job and compete for it, which is likely with this staff, then you sign a guy like McCarron who is still young and hungry.