Callahan has been impressive and could end up being a starter in the NFL some day. However, I seriously doubt any team would view him as an immediate starter or back-up this year, including the Packers. As long as Hundley is healthy and the staff is confident he can play if AR goes down, there is no reason to keep Callahan on the active 53. If Rodgers retires early or has a career ending injury, Hundley appears to be the future QB of the Packers, which IMO was why he was protected on the final 53 last year.
I would roll the dice in favor of being deeper at another position/special teams and hope to stash Callahan on the PS. But even on the PS, with Rodgers and Hundley ahead of him, what exactly are the Packers plans for Callahan?