Girls are not often taught to use their bodies in ways that release stress through athletics. And of course there are all sorts of girls with all levels of athletic skill. That is why, if a young female wants to play softball, there needs to be an outlet for her, whether she plays with the boys or belongs to an all female team.
Softball is an intense sport that varies in quite a different way than baseball. If girls prefer softball than by all means they should play it to their hearts content. However, if they would rather play baseball instead I believe they should still be allowed to partake in that as well.