Creating an unnecessary backlash
I favor criminal justice reform, prison reform to protect basic human rights, and efforts to lessen mass incarceration. The proposal to give people in prison the right to vote undercuts all of the aforementioned efforts because it creates a backlash. It allows even reasonable advocates of criminal justice reforms to be lumped in with "thug huggers" who want to allow terrorists and mass murderers like Dzhokhar Tsarnaev to help direct the future of our nation.
It is perfectly reasonable to allow felons who have served their time and "paid their debt to society" to vote. Once they are out of prison, it makes sense to restore their voting rights as one means of re-integrating them into society. A felony record often makes returning to a normal life difficult, so we should make things easier where we can. This benefits released prisoners, and also benefits society by reducing recidivism.
Currently incarcerated prisoners are a different story. People in prison do not have the First Amendment right to peaceably assemble, the Second Amendment right to own firearms, or the Fourth Amendment right to be free of unreasonable searches. Virtually everyone agrees that these things are reasonable and necessary, for the safety of other prisoners and prison staff. So why is the right to vote so sacrosanct, while these other enumerated rights are not?
Allowing prisoners to vote will also have negative public policy results - and this especially applies to violent criminals. Imagine someone like Willie Horton voting for a politician who favors weekend furloughs so that he can mutilate and brutally rape innocent people on the outside.
Yes, let's absolutely continue to look at ways we can reform our criminal justice system, examine and reform use-of-force policy including the overuse of SWAT teams, and especially to improve the living conditions of people who are currently in prison. Let's not undercut those good efforts with radical proposals that will only bring division and mockery and are only supported by a tiny group of extremists.