I think Bernie Madoff is an interesting example. He doesn't represent a threat to anybody (he is not a violent man. Furthermore we should be able to make sure he doesn't start running another scam again, even if he is free, since we know he is a con-man now), and if we discard vengeance as a legitimate reason to detain somebody, we then need to consider the value of detaining him as a deterrent to others.
If we are worried about psychopathic con-men who are incapable of experiencing fear, then detainment probably doesn't do much to deter similar crimes. I am not convinced that is an accurate representation of psychopaths though, and I certainly don't think all con-men are psychopaths anyway. Detaining Madoff probably does serve to deter similar crimes.
The question I have is if there are other punishments besides imprisonment that could fill that roll.