My daughter has pitched since she was 10, with professional instruction, and started playing volleyball in 7th grade, both school volleyball and club volleyball. Within 4 months (in Dec '03), she began having shoulder soreness that was diagnosed as tendonitis. She has been under a doctor's care since. She too was switched to defense only in volleyball to protect her shoulder. After fighting this problem for more than a year, and trying to pitch around/through the pain all last summer, she finally had surgery in March of this year, and will miss her entire 14U season. Her labrum was stretched out beyond what could heal naturally, and could not hold her shoulder stable.
Anyway, I believe that pitching in softball and hitting/serving in volleyball is a bad combination. The pitching stretches the labrum and shoulder muscels (you can tell this by how a fastpitch pitcher's pitching shoulder often droops lower than their other shoulder) and then the volleyball hitting/serving adds a tremendous amount of shock to an already loose joint. This often results in an "overuse injury", and yes I should have been more educated about what could happen to her and never have allowed her to play both sports, especially simultaneously. I'm sure that some girls are built such that they can handle the stress of both, but it is a huge risk that I wish that I had never allowed her to take. I really didn't think much about it at the time. She wanted to play so I let her. Big mistake. The upside is that my daughter is doing very well and her doctor seys that she will make a full recovery, and should be stronger than ever. But, she will pitch (I hope) OR play volleyball, not both. I hope that this lengthy post helps someone else avoid the same mistake.