Solitary was banned at a point in the 1800s, but was brought back as a form of reflection of their wrongdoings. Solitary COULD be a reflective time, but more often then not, people in solitary can become traumatized, mentally and develop permanent physiological brain damage which leads people to hear voices in their head, not remember how long they have been held in solitary, and hallucinate. It should be finally banned once and for all. Even the worst of the worst don't need this consequence in prison and no one should bring it back.
There may be times when a prisoner is acting out in a potentially violent way that solitary confinement is necessary. However, in most cases it just makes prisoners angry and takes away their humanity and ability to socialize. If they get out of prison, they will be more violent, so solitary needs to be a very limited option for brief periods of time.
Solitary confinement is an effective way to demoralize dangerous inmates. If anything, every prisoner should probably experience solitary for at least 24 hours to see what it feels like so as to act as a deterrent. Not everyone responds well to solitary, but it works in a pinch to keep unruly prisoners from hurting others.