The important part is that the state cannot be incentivized in any way to incarcerate people. By disenfranchising criminals the state is incentivized to make those that disagree with them criminals. Incarceration, if used at all must be a burden on the state, and therefore a last resort. Disenfranchising criminals is essentially giving the state the opportunity to pay to eliminate the right to vote. Given how tight elections can be this can be disastrous.