I agree. A child playing multiple sports in the 3rd thru 8th grades range is one part of the child's development as a human being. Sports at those ages are about making friends, building confidence and having fun. It should not be about being recruited and scholarships.
The science behind the argument for one sport or multiple sports is out there for everyone to look at. There is studies that are for and against, just like anything in this world.
I can say that at the younger ages, some of the sports, can help develop their skills that are also useful in lacrosse. For example, in basketball, boxing out for a rebound helps with ground balls, setting a pick in basketball is exactly the same in lacrosse. So there is tons of things that the kids can learn from playing other sports during the year and lacrosse off-seasons.
Playing lacrosse all year round is great for your child's lacrosse skills, but is it great for there development as a person and their exposure to other things in life? It's a question each one of us should ask ourselves and answer truthfully. If your child has no interest in other sports beside lacrosse, that is one thing, but they need some downtime away from lacrosse also.
We are responsible for the choices that we make for our kids and the influence we have in the choices they make. Keep your eyes on the prize. The prize is the memories and moments you share with them along the way.