Having criminals playing as criminals allows us to understand them better as well as the play. For one, by interviewing a man who took someone's life, it is easier to figure out what has happening in the mind of someone like Cornelius. These inmates show what it is like to take another man's life and then having to live with the aftermath of those actions until their life is over. By interviewing them, it's the same as if talking to the real Hamlet who was at one point dying for revenge.
The best thing about having inmates to meet is their variety. This means that every inmate has at least one available idea to every part of Hamlet's speeches. Hamlet questions if he is truly the one that has to kill his uncle and live with the idea of taking the life of another man. There is an inmate for that. If Hamlet is just raging in love for Ophelia, there is an inmate for that. And even when Hamlet has killed and is suffering for the remainder of his life in agony, there is an inmate for that.
There was one inmate that caught my attention that decided to relate himself to Cornelius. He had taken a man's life, and he described it as "the ghost of Hamlet's father was speaking to me through the pages." This meant that he had killed him before his time had come, and he was therefore forced to live in another world, and look at the things happening not being able to do a thing about it. That is what made him a criminal: having stopped someone's life before he was done living, and therefore altering the natural course of the actions that could have made the dead man far more useful in the world.
The last thing that caught my attention was the idea that the inmates have to show their light side if they want to act in the play. The first reason is that this play relates to them so much, that even though they can act it out as a tough person, they will have to lighten up if they want to actually understand it and reflect upon it in order to get themselves thinking about themselves as part of the play. Second, this is a play that they are acting in. They're not stealing, or killing anything, and they will have to lighten up once again (which is not necessarily hard since most of them actually have a family and are used to it).
Having people acting as themselves makes the play more realistic than if any other actor did it. It means that they can grow as they do this, and experience something that will help them come out a little more "sober". That is, if they ever come out.