Games are uniquely a tremendous exercise of the mind and body. When compared to a laboratory game designed without violence, Medal of Honor improved the vision of congenitally-handicapped children while the laboratory game did not. The author of the study speculated that the violence in Medal of Honor put kids into more of a neuro-plastic mood, while its emphasis on hand-eye coordination exercised the brain, the eyes and the hands in a way that improved senses.[0]
The game trains your strategic and tactical planning and it helps you socialize through teamwork, but that wasn't tested. Suffice it to say, the same features cannot be said to exist in yoga, television, or cooking.
Games are also universal. Girls and boys, young and old, physically fit and physically infirm, poor and rich: pretty much everyone can enjoy a game.[1] It is a form of entertainment that is capable of demanding zero to full emotional, financial and intellectual engagement.
Compare to sports, where we can barely figure out how to have men compete fairly against women. Or playing music, where an instrument represents a minimum investment that immediately makes it inaccessible to many kids. As the most successful games favor a viral, free, multiplayer and social design, games have evolved in a Darwinian way to be as accessible and entertaining as possible.
I don't think it's bad to exercise or to be outside. I just don't think it's substitutable, and I think it's trivializing just how different Civilization is from canoeing. Without a doubt we ought to do both, but you can't learn diplomacy from an oar.
[0] http://www.nytimes.com/2012/08/28/science/how-video-games-co...