One reason is fairly obvious by the existence of this thread. If you don't have this rule or enforce this rule, shady coaches/clubs can roster a team with only 9 or 10 players to play 11v11 counting on getting guest players every weekend. Also I would like to think winning the league does/should matter to kids, so fairness matters. While not the case in this situation, you could have bronze teams constantly borrowing players from gold or silver. With the exception of the UEFA Champions League, winning the regular season league is the most important achievement for every major professional team outside of US. Often a promotion is the ultimate reward if you're not already in the top league. Somehow in the US we have this MLS playoff system and countless summer/winner tournaments for youth teams..............but that's another topic.