Tibia and Fibula fracture Dr's suggest I leave him in a cast

My son is 14 yrs old and broke his fibula and tibia playing softball (his tibia in an angle). His Othopedic referred us to a trauma spacialiast. They both are saying that if we leave him in a cast he has 95 percent chance his injured leg will be shorter then the other by a bit and also that his leg might face outward. Why would they suggest I leave him in a cast to heal if they say it an be corrected with surgery? The specialist says there is no risk of harming his growth plates with surgery, you would think he would opt for surgery if he can correct the fracture with a plate. We went ahead a dicided on the surgery but I am concern if we made the correcrt desicion after being told to leave him in the cast...??