You see why clubs gravitate towards keeping the kids they know because it is really tough to make these selections. Start with the fact that you have 100 kids on very quick shifts. Sometimes play is elsewhere or you cannot get anyone to throw you a pass despite being wide open. You may also get put on a midfield line with two A team players and look like a superstar, next time up your linemates cannot move the ball and you get nothing going. Tough on all the kids who put a lot of time into getting ready and then some of it (and maybe understandably so) is just out of their hands.
That is why even warm up catch/throw and any drills are just as important as any scrimmage.