Every QB in the NFL was unproven at one point or another. You've got to start somewhere, right?
Until they prove it at the highest level, you can always say, "Yeah, he looked good in college throwing against defensive backs who are now accountants and dentists, but who knows how he'll do in the NFL?" Or, "Let's see him do it against starters rather than backups in the preseason."
Stidham's got a lot of arm talent... he played well last preseason... his teammates and coaches say he's done well in practice... his coaches say he's picked up the system quickly. When there's no longer anyone ahead of him on the depth chart, that's the kind of kid you give a shot to. My guess is that the coaching staff has a rough idea of how ready he is and where he is in his development in comparison to guys like Garoppolo and Brissett. And I think the fact that they felt comfortable enough with him to elevate him to #2 on the depth chart last year, and that they haven't run out and signed anyone to play ahead of him this year speaks volumes as to how they feel about him.