The idea of playing outside during school hours was made clearer to me in something called the "loose-parts theory". This theory is to say that playing in a more natural setting like a forest or a field with trees, rivers, or ponds has many more loose-parts than just a basketball court. These loose-parts allow the imagination to soar and begin games invented in the childrens mind.
It was while I was reading the book "Last Child in the Woods" that I learned about a Swedish study that found children on asphalt playgrounds had play that was much more interrupted; they played in short segments. But when in a more natural playgrounds, children invented whole sagas that they carried from day to day to day -- making and collecting meaning. (Source: Richard Louv)
Basically to put it short for kids playing outside in natural settings can have endless positive affects on our society that are just beging to be discovered. Unfortunately one of the reasons kids have to be on artificial playgrounds is because of the ear of law-suits and schools afraid of getting sued for having a kid really enjoy and get the benefits of being outside.