yeah but a not guilty by means of insanity ruling isn't a free ride either. you don't just "walk free", you get sentencing, and mental illness detention facilities are no ride in the park either.
would you rather have an insane person get incarcerated in the normal corrections system with no help at all? i think the personal vengeance through rotting the assailant in a cell solution is a larger burden on greater society than actual rehabilitation.
all of this being said, the argument won't work in court, he has a public defender (read: a workload too heavy to come up with a sound argument), and a history of substance abuse that will be heavily rebutted by the prosecutor. i don't know of legal precedents in cali, but i doubt substance abuse can lead to a successful insanity plea. the "i'm so mentally ill, i'm addicted to meth" line is probably going to be used, and again, i don't know about the soundness of that.
but if he has pre-existing medical conditions, and actually has "delusions and paranoid schizophrenia", then so be it, he needs treatment, not superficial feel good solutions, as outrageous that is to you.