Despite the uncanny ability of holdback complainers to hold on so tightly to a position, would any single one of their players get better at lacrosse if they went by birth-year-buddies system tomorrow? In spite of their own argument, a couple hold backs forced up would actually get better, their own kids' bracket would get diluted, and their own development would systematically slow down. Once diluted, younger better kids would move up from below, and they would quickly realize the really good kids one year behind are really freakin good too! What then? No playing up? Birth-year-buddies for life!
Once again you argue from the (weak) position that by playing down you are somehow doing the rest of us a favor and helping us get better. Ok then, following your logic, by playing down, against easier competition, aren't your kids then getting..worse? This dilution talk is just a red herring. It's not about any one single player getting better, it's about all players being compared to their actual peers, unless being compared to smaller, slower, weaker kids is the only way your kid can stand out.
