What are some pros and cons of having the insanity defense used in criminal cases?

The insanity defense is a mode of defense used by defendants in courts to argue that they were not completely responsible for their criminal actions, on the basis of a prevailing mental illness. By using such a defense, the defendant expects the court to lessen the responsibility of the criminal act and reduce or renounce the sentence. 

Different countries have separate parameters for declaring legal insanity. Although the final decision may rest in the hands of the judge and the court, psychologists and psychiatrists are often consulted during the hearings for their expert and scientific opinion.

It is important to note that the discipline of psychology doesn't particularly use the term "insane" which is more common in law. Psychologists prefer the term "mentally ill or retarded or challenged".