Probably the noblest and most humane purpose of punishment in the criminal law is rehabilitation. When a citizen's criminal tendencies are "cured" (in a manner of speaking) so that he or she never has the urge to commit crime again and, even further, becomes a productive member of society, then society is not only protected from future harm but it's also made richer by the successful re-entry of one of its members. It's a win-win situation in which both society and criminal offenders benefit.
Idealogically, rehabilitation is a very sound goal for punishment. It's pleasant and beautiful to imagine the successful general rehabilitation of society's criminals. If only adult criminals could be successfully rehabilitated, then the phenomenon of crime could be all but eliminated, and criminal offenses restricted from then on to juvenile delinquency and the occasional act of passion.
Ah, if only. While few seriously argue against the utility of reforming criminal offenders, there are powerful arguments against placing too much importance on rehabilitation, not the least of which is that it tends not to work. In 1994, over sixty percent of criminal offenders who were released from U.S. correctional facilities were arrested again within three years or less. Fifty percent went back into the system. High recidivism rates are a powerful argument against the effectiveness of rehabilitation in the criminal law. It is time-consuming and dubious effort to meaningfully reform serious criminals, and it costs more for tax-payers. However fine and noble the idea of reforming criminals into productive members of society may be, the statistics alone speak out strongly against the attempt.
On the other hand, it is probably a bit much to argue that criminal offenders are fundamentally unworthy of the efforts of rehabilitation, and that it's good for them to suffer for what they've done without any help or reprieve. Perhaps. In the real world, many criminals may be truly un-reformable, and any attempt to rehabiliate them would be a waste of effort and resources. Also, the pain of crime victims and their loved ones cannot be ignored or reasoned away, and to deny them some feeling of satisfied vengeance could be seen as an abject failure of the justice system. But, all things considered, it is at least feasable for a society that cherishes the precept "innocent until proven guilty" to some day place equal value on the precept "reformable until proven otherwise." Of course, the only way to prove this is to try.