There are a couple of comments amongst posters that I wholeheartedly agree with. They would be the following:
a) kids who throw MORE have fewer injuries
b) specializing in one sport could be a meaningful factor
I think as parents, with all the talk about pitch counts, they restrict their kids from throwing often enough. I emphasize the word "throwing". Not pitching per se. A common parent of a 9 year old, hears all of the chatter about arm injuries, and they naturally want to protect their kids, and therefore advise their children to take rest too often. I also have witnessed too few kids who stretch their arms out properly when warming up. In this instance, I am referring to long tossing. Throwing regularly, year round, with the implementation of long tossing can cure a lot of ills.
I also believe that kids playing only sport can be a detriment. When one plays multiple sports, they are forcing themselves to work muscle groups that they otherwise would not. This strengthens and trains the entire body, rather than isolating one group. Every day, we see infielders who do not have the strength in their legs to utilize their lower half properly when receiving ground balls. (they don't move their feet properly, and don't get the butt down.) Basketball, for example, is a great crossover sport to help train that muscle group.
JMO