When you work out every kid at QB, then you also have to work every kid out at all of the other positions if you are going to field a team. I don't think most teams practice more than a few hours per week. It would be incredibly hard to teach kids blocking assignments, handoffs, routes, passing mechanics, tackling techniques, pass coverage techniques, etc. with that amount of practice time. It really only would work under a 7 on 7 scenario.