Should Prisons Be Only for Psychopaths?

The debate about mass incarceration has led me to think about who should be imprisoned.

Of course people should be punished for their crimes.

However, I think prisons are an ineffective way to fight and treat drug use.  If we prevented drug use better, and treated drug use separately from prison, then rehabilitation for former drug users would be much more effective.

This would leave the prison system for the hard core of career criminals and conscienceless psychopaths.  We would still have prisons, but would no longer use mass incarceration as our main tool of social control of poor and lonely men.