socal,
First of all winter ball is usually relatively weak so you don't always get a true picture of how players are going to handle stronger competition.
Second players can develop pretty quickly at this age so you may end up overlooking someone who could help the team.
Third, players get hurt and it sure helps to have someone who has game experience ready to step in.
IMO, the best approach is to get the bench players some playing time in non-league games. That doesn't mean a lineup of all bench players by any means. It just means giving them a chance to show what they've got. In league, you go with your best at any given time.
Pitchers can be a bit of a different story in HS. Pitchers need development. Everybody has heard that Koufax became a great pitcher after he learned to take something off his fastball. Not everybody knows that he felt it would have happened a lot sooner if he'd gotten innings. Once again you don't want to take a hard thrower who gives up a lot of walks and start him in league games if you've got a more reliable kid, but if you get the hard thrower innings in non-league games it may pay off come the playoffs.
The other thing is showing the kids that hard work does pay off and that's a good reason to give the players who earn it a shot in the non-league games.
Unless your team is gunning for a national championship who cares about non-league games? (OK, any coach cares but you can still try to win with a mixed lineup.) If you are trying for a .500 record to make a wildcard game by winning non-league games then you aren't going to go anywhere in the playoffs anyways.
