There's a big difference between allowing people in prison to vote, and in never allowing them to vote again after they are released. People in prison by intention don't have the same civil rights as normal citizens, and so withholding the right to vote doesn't seem unreasonable. Withholding it in perpetuity seems outrageous.
I don't think it's unreasonable to suspend the right to vote to some (or indeed to most) prisoners.
But I also don't think it's unreasonable to allow some prisoners to vote, especially where they are imprisoned for short sentences and where it is in the interests of rehabilitating them to do so.
I don't see the point, though, in taking the right to vote away from prisoners.
It's not like stabbing someone makes your opinion on taxes less valid. Or drug dealing makes your opinion of the education system invalid.
Even if you rob someone, your opinion that the poor (a.k.a. probably you) should be financially supported by the government is not less valid.
There can a point be made that you're not exactly participating in society while you're in prison, so if we act like politics always perfectly reflect people's current needs and don't cumbersomely shape over decades, then you could argue that they shouldn't be allowed to vote over other people's society, therefore should only be allowed to vote when they're going to be released in the next legislature period. But that's just not the case. A moron in presidency can and will negatively affect people's lives for a long time after he's gone.
Lastly, I suppose, if you view prison primarily as punishment, then taking away their right to vote just extends that.