I think you're misunderstanding my point here. If Brosmer were a legitimate, starting NFL QB talent RIGHT NOW, I'd agree with you. Chances are that he's not at this point (and may not be at all). But what if he looked even marginally better than JJ? Team still isn't going to make any noise this year. Do you even want the questions coming up in the locker room about whether JJ is the guy or not? The coaches know what they have, and I'm guessing that even if Brosmer is an upgrade at this point, the margin is minimal to the point that they don't even want the potential for controversy.
It's all speculation. I'm hoping that Brosmer gets in the game. I'd love it if he ended up becoming the Vikings franchise quarterback. But there's a reason why the Vikings are choosing to roll with Wentz, a guy that was basically out of the league before we signed him, instead. And I'm not sure it has anything to do with Brosmer's readiness. His play in camp and the preseason is why the Vikes traded Howell. So they clearly think something's there.....but aren't going to play him now?
Again.....I'm just guessing that since this is a short term injury for JJ, that they don't want even a hint of controversy dredging up since they'll undoubtedly be going back to JJ when he's healthy.