I like the idea but I wouldn't think thats possible because there are a lots of criminals who like to take advantage to the system where he/she will pretend to be an insane after slaying/killing the victim. So the criminal will likely admitted to a mental hospital for evaluation instead of putting that person to jail. Jail is pain and its not a good place to be especially if you are being mixed with a bunch of hardcore criminals. So as like i said, a mental institution is not a good option for real criminals. So, yes the Society need Prisons.