The left and right hemispheres of the brain are connected by a bundle of fibers called the corpus collosum. Ambidextrous, left handers and cross-dominant people have symmetrical brains as opposed to right handers, who have larger left hemispheres. Cross-dominant, lefties and ambidextrous people also have a stronger corpus collosum which lends itself to better communication between the two hemispheres. It seems like better communication between hemispheres would be an evolutionary benefit.
So, I am curious as to why a symmetrical brain would have a higher rate of mental illness than a right handed/left brained person, as reported in this news release by Imperial College London: Mixed-handed children more likely to have mental health, language and scholastic problems, say researchers