The April 2010 Scientific American has an article by Michael Gazzaniga (a famous cognitive neuroscientist), "Neuroscience in the courtroom", partly about this issue. He writes:

A neuroimaging tool or method that could reliably identify psychopaths would be useful at the sentencing phase of a trial because it could help determine whether the defendant might deserve [my emphasis] medical confinement and treatment rather than punitive incarceration.

This is what we see most commonly in debate on the issue: Not a reasoned defense of any of the various positions, nor an argument why sentencing in a particular case should be done for punishment, crime prevention, or deterrence; but blithe unawareness that there are any considerations other than what the defendant "deserves".