Not letting convicts or ex-convicts vote is discriminatory. Why should they not be allowed to vote, when their quality of life is affected by politics too? If their lives are not worthy, then why are we feeding them, housing them and providing them medical care? Since voting is an essential function of being a citizen of the country, it is as if convicts are not considered valid citizens, in which case perhaps they should all be exiled rather than fed and sheltered.
Furthermore, not allowing them to vote can effect, to some degree, a self-perpetuating tyranny of that faction of people who do not commit so-called crimes—that is, the same faction of people who, by voting, can determine what actually constitute a “crime” as well as what the punishments are. Some things may be able to continue to be regarded as crimes that are actually unjust, or not in the popular interest, to be considered crimes, because those who commit the crimes are barred from voting.
Here’s an example: let’s say 40% of people think smoking marijuana should be a felony, while 60% think it’s just fine. The 60% who think it’s fine are convicted for smoking marijuana because it’s illegal, while the 40% not in jail continue to exclusively vote on legislation and legislators that keep marijuana being a felony. The numbers chosen here may be a hyperbolic extreme, but the point is that the general effect they illustrate may actually occur to some degree or another.
Another important example is prison reform. The conditions people have to live under in prisons are deplorable, but if convicts and ex-convicts can’t vote, then there’s nobody to stand up for the people subjected to the unacceptable conditions of the prison environment.