You do so to give them some game experience. Jim Harbaugh played his 3rd string QB last year. Why? Because he knew he had a senior starter that was leaving the program, setting up this seasons battle for the starting spot, while allowing both backups to get some needed real game PT. Doc knew his 2nd string QB was done after the season, so why use him in mop up time, when he could have allowed the projected backup for the upcoming season to get some playing time. He could have at least played him during the VMI, Cincinnati, and UCF games. Is Doc aware of the new red shirt rule that went into effect a couple years ago?