I think that not only should they be allowed to work, they should be forced to so. It is ludicrous to me that these people are holed up in a penitentiary somewhere when they could be out cleaning up garbage or doing other unskilled labors. Things our government pays others to do could be done for free with the use of prison work teams.
And as you said, it could perhaps influence life after prison. When a person is forced to work, perhaps a little work ethic will rub off on them and perhaps they can develop some kind of ability to hold a job when/if they get released from prison. It also stymies the idle time by which these people have to plan attacks on other inmates or guards.
These people may be tired and actually sleep at night if they were given the opportunity to work for 8-10 hours a day. They will have their mind taken off of trying to kill this inmate or that guard because they will potentially have other things to think about.
I think full on that inmates should be allowed to work, and forced to in some cases. Work does so many things for people including raising self-esteem and self-awareness. It is something to be proud of most of the time and this is one of the things inmates need. They do not feel they can make it doing real jobs so they turn to lives of crime perhaps. Well these kinds of programs would afford them the ability to work and thus understand society in general a little bit better, which beats the hell out of sitting in the prison all day and lifting weights and getting angry about the situation. Anger breeds hate and when these people have no other outlet for their energies, it will come out as that.
Edit: Sherryline, you are right that prison is about punishment but it is also about rehabilitation. And the rehabilitation part is not working because inmates sit idly for 20 + hours a day. When nothing constructive is being done, then the chance at rehabilitation is slim to none. Showing these people how to work and letting them see the self-worth that comes from working could potentially be a big step in re-introducing them to society as productive members.