At 10U I am a big fan of girls playing multiple sports. Get her into volleyball, basketball, lacrosse. I would worry about burning her out if she is playing or practicing softball 12 months a year at 10U.
Absolutely! However, I'll offer a word of caution with this as well.
Since day 1, we've told my daughter "try it!" If you like it, great. If not, we won't do it again, but at least you tried.
Our problem comes with the fact that our daughter LOVES everything (with the exception of dance - that was a 1 and done)!
So, now, we have pretty much year-round competetive gymnastics, Fall and Spring soccer (if you play club, you can't just do one of the seasons), Spring softball, and Fall cheerleading. Luckily, she chose to put the cheerleading on the back-burner this coming fall, because something was going to have to give.
I think we can make it through 1 more year doing this, but after that she's going to have to make a tough choice. And at 8/9 years old, it sucks that she'll have to do that. The almost "year-round" epedemic in almost every sport is horrible, IMO.