Would most people be better off undiagnosed? When it comes to mental "illness", often the only (or at least the best) treatments are behavioral therapy, in which the "illness" is trained away.
I'm sure you posted the above in good faith so I've deleted my original knee-jerk response to your comment. Instead, let it suffice to say that behavioral therapy is only appropriate, suitable or relevant for a very small percentage of patients that fall under the encompassing umbrella-term of 'mental illness'. One does not train away the illness of what we currently label as Schizophrenia, Major Depression or Bipolar Disorder.