I think prisoners should have the right to vote. To deny it undermines the very idea that we live in a democracy. Democracry means everybody gets a say, not just those we like.
I think people tend to get worked up over 'criminals' who have committed crimes against people, e.g. if someone has mugged someone, they are A Bad Sort who should be locked up, denied TV, fed gruel and preferably spat on occasionally. The fact is that prisoners and the reason they are in prison are much more complex than that. Having worked in a prison, I saw people who were there for non-payment of fines that had been issued in error, for defending themselves after years of domestic violence, debt/fraud that their partner ran up and then absconded, and as cory says all sorts of ASBO related reasons. This is not to mention people who are locked up for having done nothing at all.
Until we can be sure that everybody in prison has consciously decided to opt out of society by committing a heinous crime, then we have no business denying them the vote, IMO. If we decide that prison means no vote, then fine, but we cannot say we operate a democracy. Democracy means you don't get to choose who has a say. If we do, where does that end? In almost every example in history, where the right to vote has been denied, it has later been universally agreed that it was wrong to do so.
I also think it's a valid point about governments getting to choose what is a crime. The example of a military junta is very extreme, but again, there are far more nuanced examples of people being locked up for things that are 'frowned upon' by popular thought in certain countries at a certain period in history, which later or from the outside looks extremely barbaric.
D