ASA "pinch runner"

[Mikie]

I need some clarification on a rule. Let me set the situation.
Teams have already played 7 innings with one courtesy runner for each team. Score is tied and the home team is batting. No one is on base and there are no outs. The batter come up and hits a base hit stays at first. All game, this guy has had a courtesy runner who was the last out. The last out was a runner who does not run so well, so the team puts in a individual that had not played all game off the bench. That person flies around after a ground ball was hit and scores. Game over.

I spoke with one of the coaches after the game he said that they can replace any person on the field with someone on the bench. Where as I do agree to that comment, my question is, once the person that is being replaced hits, can you replace that person? If this is the case, why wouldn't every team line there benches with fast runners that can hit and let big slower running hitters hit and then leave the game? I have been in numerous leagues and according to ASA rules, a batter has to run the bases unless they request a courtesy runner or are injured. I understand we don't play fully by ASA rules, but if this is the case, what is the point of the courtesy runner?