Here's an example.  It is almost axiomatic that a person with a hemoglobin of 4 who refuses a transfusion is incompetent.  Nothing they could say could possibly justify their refusal-- except that they are Jehova's Witnesses.  I observe psychiatrists routinely dismiss these evaluations, and they're inwardly proud of their progressiveness: "No, he doesn't need an evaluation, he can refuse, that's his religion, his personal beliefs."   1) This seems pretty preposterous; are you saying cultural dictums outweigh personal choice?  A person can't choose, but his culture can choose for him? 2)  Is it possible that even a Jehova's Witness might actually want a transfusion but feels pressured by his peers to refuse?  Isn't this more properly the purview of psychiatry-- helping people deal with the pressures in their life-- rather than the lazy preemption of personal choice?  3) Is it possible-- maybe, I'm just asking-- under the right circumstances-- that even a Jehova's Witness could be incompetent?  I'm just speculating-- that their refusal is due to delirium?

And let's not discount expediency, whereby the doctor simply doesn't have the time to wait for the patient to get on board the treatment trolley.  e.g. primary medical team wants to put a guy on IV antibiotics, but the patient refuses, he wants to go home.  Most of the typical debate surrounds their mental state, and not the following simple question:

ME: Well, if he refuses, what are the alternatives?
THEM: We'd have to discharge him on oral antibiotics.
ME: Would this work?
THEM: Well, it's not ideal.  There's a good chance he'd end up back here in the hospital in a few days.
ME (not punching anyone): if he has someone at home who can help take care of him, etc, he, unfortunately, (squeezing the thumbtacks in my hand) has the right to refuse.
THEM (frustrated, angry): Fine.  Whatever.  They have to sign an AMA discharge, and know that we're not responsible for what happens.

Note the final aggressive maneuver.   It's the only thing they can do to "punish" the patient-- for not doing what they wanted.  It's more clearly seen when I say this:

ME: Unfortunately, (tacks in hand again) his refusal doesn't discharge our obligation to treat. He'll need an outpatient appointment within a day or so.

THEM: No, I'm not doing that.  If he doesn't want to follow my prescribed treatment, I'm not going to alter my schedule for him. 

ME: Unfortunately, if you were ready to find him incompetent and keep him in the hospital, lawyers won't understand why you didn't follow such a sick person more closely as an outpatient.

I always blame lawyers, not because they are to blame, but because it's the only thing doctors really understand.  But either the patient is really sick, and we can have a discussion about incompetency, or they're not, and we shouldn't be having the discussion.

Another piece of idiocy I get from other doctors is this: well, would you want your mother to refuse?  Exactly what in the word mother presupposes an inability to make decisions?  But they miss the crucial point: that even if she was incapacitated, the person who would decide for her would be-- me.  Her family, or, if necessary, a judge.  Substituted judgment.  Consequently, the family would have to be involved anyway-- so involve them now.

Rather than force a family member to have to go against her wishes to do what is in "her best interest," it would be better to help them convince her to do what you think is best.  The time spent during the consult should be devoted not towards assessing their competency-- let's face it, if you can't tell in five seconds that they're incompetent, then they're probably competent-- but towards educating the patient, conveying some empathy and assurance, and getting them to choose for themselves what is best for them. 

Because some day in the future they're going to get sick again, and they may be much less interested in coming back in for treatment if they think we're going to just do what we want to them.