Firstly, and i don't know if this is the case in the states, but in Canada where I currently reside, there is a significantly high rate of people who are put in jail because they have been falsely accused. Secondly, would this include incarceration while awaiting trial? If you are truly innocent until proven guilty, than pre-trial incarcerants should be allowed to vote.
Plus I disagree with Derrik. China has the world's most severe forms of capitol punishment, and public executions occur on a highly regular basis. If Capitol punishment were truly a deterrent, than wouldn't these executions be less frequent, rather than more frequent?
In Canada, we do not execute anyone, and we have a significantly lower crime rate than almost any given U.S. state, including ones where capitol punishments exist. So do most European countries. We also have a lower rate of repeat offenders. Granted there are still flaws in our system, when we consider what most repeat offenders crime is, and I do believe we should crack down on them.
In the end, I believe voting in jails should be mandatory, as it encourages people to become socially active and better model citizens upon release.