Methinks you have had little involvement with the criminal justice system.
One of the reasons so many young people "go about their dishonest, illegal murderous buisness" is because they feel no connection with the society on which they prey. Prison is not just about punishment - if it is to work it must also be about rehabilitation, and giving people a sense that they have a stake in society is part of that process.
I can see practical difficulties in enfranchising prisoners (particularly in terms of the constituency in which their vote would count), but I support the principle.