Agreeing with ghostpaw. Should only be on their back if you need to perform CPR really. Or shock - on back, raise leg.) Other reasons I can think of involve dealing with bleeding....

Although from my time in St John we did get storying about people getting shirty about cracked/bruised ribs, We were also taught if the casualty is wearing glasses put them somewhere safe as they have had complaints about broken/scratched glasses! It was a small percentage of treatments though.