Mental illness is often used as an egregiously loose label, & sometimes psychiatry has been used to silence political oppositions, but there is no denying that there are people who actually suffer from mental illnesses, sometimes so much so that sufferings lead to the destruction of sufferers themselves and/or others.
While it is valid to criticize the idea that doctors, merely by virtue of being doctors, always know the best interest of patients, it is futile (not to mention "bourgeois"!) to present the model of "the informed consumer" -- the patient who knows very well what s/he suffers from; what treatment, if any, s/he needs; what treatment, if any, s/he wants; how to evaluate varied competences of different doctors; how to weigh the merits and demerits of different treatments; and so on -- as the alternative to what you criticize. Some people do fit the profile of "the informed consumer" by virtue of their circumstance (biological as well as social), but it is clear that many people do not. For instance, a very old person with a severe case of Alzheimer's disease (whose symptoms include memory loss & problems with reasoning) is helpless without a care-giver making some decisions on his or her behalf: "Since no two people experience Alzheimer's disease in the same way, there is no way to predict what your role as caregiver will entail. Responsibilities can include making important legal decisions, managing changes in your loved one's behavior, and helping him or her maintain hygiene" (at <http://www.alz.org/caregiver/guide/>). The same goes for a person with a severe case of schizophrenia, who may not be treated without involuntary admission.
More people oppose the execution of the mentally ill or retarded than oppose capital punishment tout court, and that's because some forms & degrees of mental illness & retardation make sufferers ethically irresponsible, that is, not in possession of the degree of mental competence that would make them responsible for their actions. Were everyone equally mentally competent, regardless of his or her age, illness, etc., it would be correct to treat everyone in the identical fashion before the law, but it is not.
Also, you have opposed the right to assisted suicide, and I believe your opposition is rooted in a reasonable assumption that not all individuals are in a position to make an informed decision as to when & how they want to die & to insist upon it (even in the face of implicit or explicit oppositions from others) & that an irreversible decision may be wrongly made by others who consciously or unconsciously wish them dead. On this question, the model of "the informed consumer" doesn't help you make your argument at all.
Yoshie