I didn't have classmates who routinely crawled under their desks, cursed out teachers, or banged their heads against the walls.
My child had to deal with this type of unacceptable behavior everyday.
I doubt the disturbed children benefitted much from the experience.
I know the “normal” children didn't gain anything constructive, other than pure disdain for their supposedly superior adult teachers, who tried to force them to pretend to tolerate abnormal behavior.
I think you have hit the nail right on the head. Why this helps the "mainstreamed" child I will never know, but I do know it hurts every other child in that class.
Best of all, most times it is not permitted to concentrate all of the "mainstreamed" kids in the same class. They have to be spread out so as to disrupt the maximum number of normal kids.