The issue here is those who are currently NOT qualified to vote.
IMHO, this seems like one of those dead-end causes that we on the right too easily wander into, like flag-burning amendments and such.  If we take the proper perspective of how pernicious government is, we see that it's definitely in the government's interest to keep former felons from voting.  That gives terrific incentive to create ever-more felons.
As to Michigan's policy of not allowing those currently in the slammer from voting, there seems little to worry over one way or the other.  Is there some mass movement among current jailbirds to vote?  And who are we afraid they're going to vote for, anyway?  Overall, I'd say we should be doing all we can to encourage more, rather than less, suffrage.