With all due respect, what teams do you guys play for? Go tell Wando or Irmo or Riverside or BE that high school soccer is just an after school activity. The notion that soccer is just an after school activity at the high school level is ludicrous. I can name about 50 schools right off hand where soccer is not just an after school activity but a passion and is played at a high level. Yes, teams do use subs to give starters a breather or to melt the clock, but that means that coaches are free to use all the players on their bench if they choose. If you limit subs then less skilled players will have even less of a chance of playing. It will also be more difficult to get younger players valuable varsity playing time.
Also, if a very skilled team is playing a less skilled team and gets a big lead, then they usually like to empty the bench and get younger or less skilled players some valuable playing time while resting starters. If you limit subs, that will mean that more starters will have to be in for the duration of the game which will lead to even more ridiculous score lines. Look at how many times we talk about 10-0 score lines on this message board-- limit subs and prepare to discuss 20-0 score lines. Also be prepared to discuss more career ending injuries as those teams losing 15-0 get frustrated and start whacking opposing players.
Also, unlimited substitutions allow for schools where soccer is just an after school activity to allow more players to participate in that activity. If you limit subs, less skilled players on less skilled teams will see far less playing time. Sub patterns, when to rest starters and how to maximize your bench is all part of the high school game.