I think yes, they should, for the simple reason that they are still a part of the system. Prisoners are still citizens even if they've committed a crime and shown resentment towards that system and society. Prisoners still have rights and should not have to forfeit their intelligence or political nous simply because they broke the law.
Not allowing a prisoner a vote reflects badly on society and government. It is tantamount to saying - you're no good, you're useless, you may not even exist.
I cannot see the right in any argument that says: by breaking the law you are no longer considered a bona fide citizen. It's not about respect, or love, or denial, it's about being a human being in a mature political democracy.
I say everyone is equal in this, criminal in prison or free citizen.