I have moved into a work setting where there is much talk of "clients" and "consumers" in lieu of "patients" as those seeking mental health care (Shrink Rap recently alluded to this time-honored issue). I'm no Latin scholar (the title of this blog notwithstanding), but my understanding is that "patient" derives from the Latin referring to one who suffers or endures some condition, and not necessarily with any connotation of passivity or subordination.
It seems to me that suffering is the key concept here. A consumer utilizes goods or services in a discretionary sense, and one could argue that this could include even "somatic services" such as a manicure, a massage, or even (illegally in most places of course) sex. A client is in need of someone, such as a real estate agent or an attorney, for representation or guidance in a potentially complex social or financial interaction. But a patient is someone who is either actively suffering or under threat of potential suffering and death in the future; in the latter sense we are all of course "patients" at some time or other.
To be sure, one could argue that someone presenting for an annual physical may not be actively suffering at all (until the rectal exam), but however much we might try to talk about "wellness," it remains the case that doctors' offices make people nervous because they are reminders of vulnerability and mortality. Doctors (including, indirectly at times, dermatologists, radiologists, and, yes, psychiatrists) are entrusted with matters of life, death, and suffering, so I would argue that anyone interacting with a physician in a professional setting is in fact a "patient."
Psychotherapists of various kinds occupy an interesting middle region here. Certainly people who consult psychotherapists are in some kind of distress, more so on average, presumably, than those who consult real estate agents or attorneys. And some cases of psychotherapy are so existentially intense (or the symptoms involved so severe) that patienthood seems appropriate (after all, therapists are called upon at times to hospitalize people, and it sounds ridiculous to talk of consumers or clients being committed to the hospital). However, some cases of psychotherapy (for instance, marital counseling) may be sufficiently narrow, and sufficiently bordering on what some might consider "problems of living," that "client" may be a suitable term.
I for one think that "consumer" is never appropriate in clinical contexts--save that for the accountants and insurance companies. To refer to patients as comsumers is to minimize their distress, both actual and potential; it is to imply that they are suffering no more than someone stopping by the convenience store to consume a six-pack.