Some criminal acts are serious enough to warrant depriving people of more than just their liberty. It's the blanket ban approach that seems wrong.
Many of the inhabitants of our penal system are as jim described - first time offenders, white collar criminals,etc, so taking away the right to vote seems a bit excessive, to me. And we should also be considering the balance of what we are trying to achieve with prison - safety of the community at large, punishment of the individual who has broken the law is a given, but if we wish them to become productive members of society once they have served their sentence, rather than simply going back to a life of crime, there has to be an element of rehabilitation too. And voting is a civic duty so could be seen as part of the rehabilitation.
It does mean finding some way of separating the minor criminals from those undeserving of keeping such a right though, and probably the easiest way of doing that relates to the length of their original sentence.