I think it's only because he knows Martz system. This isn't the same as the Jeff George signing in 2004.
This, evidenced by the reports coming out of mini camps (NOT the reports the Bears give) that absolutely NO ONE knew what was going on when the offense was on the field, and practice had to be stopped some 8 times one day so people could be re-positioned and shown how to actually do the things expected of them.
Despite him being a coach in training pads, I don't know if I like our organization signing Quarterbacks out of retirement. It's just the notion of us grave-digging for a 3rd/4th string signal caller.
There are guys who if they had to get in a game, would be able to produce, sitting at home on their couches.
I get what you're saying, but I think it's important to remember that backups and depth-chart fillers at the QB spot aren't the same as bench players or relievers in sports like basketball or baseball.
So signing a guy that has almost no business playing football again (either because of past injuries or current ability level) to be the 3rd stringer isn't necessarily as bad as a baseball team signing a similar type of player to their MLB roster (Omar Vizquel, Randy Williams, etc.) because the latter will actually be needed to contribute throughout the course of a season, while the former won't be needed to contribute that much--if at all--should things go according to plan.