If the criminal is violent and causing risk to other people, put them in solitary. When it comes to others safety and well being vs the delicate mental state of a violent or unruly offender, I'm on the side of keep them locked away from others. That is until a better alternative is available, which there doesn't seem to be at this time.
At this stage of punishment, these crooks are never meant to reenter society. They may be dealt with any punishment that is not cruel and unusual, but may go with either or. They should have to be kept from all others for valid reasons, and thus it is valid, in conclusion.
Solitary confinement is mentally damaging. I've watched interviews of inmates formerly put into solitary confinement and it just sounds like a horror movie.
People have gone completely insane from solitary confinement. This type of punishment is torturous, making it unconstitutional due to our 8th amendment. Solitary confinement does not reduce prison violence, and doesn't reduce violent crime rates at all.
Solitary confinement is now used as punishment for those who simply break prison rules, such as using profanity. It's not just used for violent criminals now. Some of these criminals in solitary are just arrested for drug charges; how stupid is that? But that's a whole other topic.
When inmates finally get released, they are mentally ill and almost insane and have no way to conform to society. This can leave them homeless and actually deprives ourselves of a potential income taxpayer.
Lastly, solitary takes up more space in prisons and costs more. It costs our taxes $120 a day in solitary while a general cell with one or two more people costs $70 a day. Abolishing solitary confinement can take the weight off our tax money. Since solitary confinement only has one person per cell, this takes up more space. If you're seriously worried about prison overcrowding, be concerned about solitary confinement.
Solitary confinement for a few hours is fine. But for days, weeks or even months on end it is very psychologically damaging.
Many people have actually gone totally insane as a result of being placed in solitary confinement. A punishment is meant to deter someone from doing the wrong thing again. If they go insane it's not going to be much of a deterrent.