Banning prisoners from voting, in no way, supports rehabilitation. It is simply an arbitrary punishment. Why should they not be able to vote? After all, their lives are greatly affected by government decisions. The majority of them have committed crimes, but they are still citizens. We don't lose the privilege for speeding or J-walking. What makes any other crime different?
Voting is designed to allow the wisdom of crowds to move a group of people in the direction of its collective best interest. When a person commits and is convicted of a serious crime, they have acted in a manner opposing that spirit -- they are acting to move society toward their INDIVIDUAL best interest, to the exclusion of all others. When a person makes the decision that their own interests trump those of the greater group, to the extent that laws and rules are ignored, the right of that criminal to participate in the future decision making process of the group should be revoked. This is anything but arbitrary.