Eh, it's useful in that Jared can cross a lot of names off the list of people he trusts.
Yeah, every year there are guys that weren't that impressive in camp, but then were in the pre-season games. Some guys just need that extra competition to bring it out in themselves perhaps.
You also can't judge the reliability/trustworthy aspect that you reference from practices. If they have a chance of playing in the NHL, then it's helpful for a coach to evaluate them in that area during a game. Hard to replicate the situations, and the intensity, in a practice setting.
Behrens is this year's example. Bednar said he didn't really like his rookie camp, but thought he really improved his performance once the pre-season games started. That definitely helped his callup chances this year.