Why is this bad? If you're going to have a truly fair system EVERYONE should be allowed to vote.
Not that I support democracy in any way given the consequences of it right now, but there you go.
Because prisoners would then have a disproportionate influence in the district the prison happens to be located in.
As opposed to now, where legislators get elected from small districts because of disenfranchised prisoners.
Perhaps we can come to some sort of compromise. Say prisoners count as 3/5ths of a person, for example.