1) This was the norm in the first prisons in the country during the 18th-19th centuries, which resulted in a recidivist rate of around 10%, as Skotnicki notes:
“The central coordinates of the separate and silent systems, silence, work and moral/religious training, were not found to be ineffective as formal guiding principles. It was the conditions surrounding their implementation, particularly in New York with its inordinate demand for financial stability, and its unfortunate misuse of corporal punishment, that eroded their value. Despite the fact that the statistical methods utilized to determine the effects of the penitentiary discipline on recidivism often missed reconvictions in other states, were certainly tinged with ideological bias, and cannot be necessarily equated with penal methods, it would be a mistake to ignore their findings out of hand. The chaplains who conducted them were not always blind supporters of the administration. History provides clear evidence that they were willing, to a significant degree, to critique institutional practices. Still, their data concerning recidivism was most favorable. Prior to the Civil War, the rates of reconviction were consistently less than 10%, with the data from the Eastern Penitentiary being the lowest.” (Skotnicki, A. (2000). Religion and the Development of the American Penal System. New York: University Press of America. (p. 145)
2) The rise of legal advocacy groups and various prison reform movements have worked to reduce its use—based primarily on the argument that it caused mental illness among the prisoners during that early use—and it is being reduced even more, as this story from USA Today reports.
“State prison officials are reducing the number of offenders in solitary confinement — once among the fastest-growing conditions of detention — as budget pressures, legal challenges and concerns about the punishment's effectiveness mount.
“States such as Mississippi, Texas and Illinois have decreased the number of inmates in solitary confinement, a dramatic acknowledgement, analysts say, that states can no longer sustain the costs of hard-line criminal justice policies.
"The whole philosophy of being just tough — locking people up and throwing away the key — has not solved the problem," said Texas state Sen. John Whitmire, Democratic chairman of the Senate Criminal Justice Committee.
“Decisions to return dangerous inmates to the general prison population anger some prison officials, who say the changes could threaten the safety of corrections officers and other inmates.
"The departments of correction are rolling the dice with public safety. ... This is going to blow up," said Brian Dawe of the American Correctional Officer Intelligence Network, an association of officers.
“The number of prisoners in solitary confinement — typically locked away for 23 hours a day — grew 40% from 1995 to 2000 when there were 80,870 segregated inmates, a study by The Commission on Safety and Abuse in America's Prisons found. The overall prison population increased 28% during that time. Isolating prisoners, the private study found, is often "twice as costly."