That's exactly why you take him, imo. Especially if it's day 3.
You can teach mechanics, understanding of the game, and to some extent, processing speed of the game. What you can't teach, like speed, is arm strength. All it takes is the right coach to turn a QB into a gem (e.g. Purdy). Maybe Kubiak is that guy, maybe not. But I don't think he's not worth a shot.