Should the state train prisoners to do high skilled jobs?

It seems there are a number of states who are using prisoners to reduce deficits. They can pay prisoners a pittance and force them to work as long as they want.

I know that in some states, prisoners are not allowed to do work that civilians do. Labor unions have made that happen. But that doesn’t seem to be all the states.

If prisoners are used as nearly free labor in order to save the state money, will that increase unemployment? Is it ethical? Is it good policy? Will people want to get more low-security prisoners to do more work for free? Will this offset the cost of keeping them in prison? Is this a good deal?