I don't see how he can be in football shape after 5+ years out of the league. He could barely read defenses before, hence all the running yards he had, and needs to learn the playbook. As a back up he might be okay for a couple seasons. But do they want the headache if he starts his protesting again? Aside from all that, what sticks with me mainly is how he quit on the team when he got benched. Didn't go to meetings, etc. Turned down the contract option for his last year. Then the protesting started soon after.
He was brilliant at running the Pistol, which is fine for college but would eventually get someone killed by the much faster NFL defenses. It got them to the Super Bowl that year though.
As far as reading defenses, he could do that fine, just not fast enough before the pocket collapsed if the first option was covered. Turns out you have to be intelligent and think quickly to be a QB in the NFL.
Even if he could play at this level, who needs the media circus that would surround him. Fuck this guy.