The patients are scared. Doctors are playing on those fears, acting on the assumption that fast and aggressive treatment is the wisest course all the time. The patients are reacting out of fear, not getting second opinions, and are hurried into the operating table to be cut open.
That is the norm. I have seen it happen.
It is my personal opinion that, no, most doctors are not offering complete information. So your argument, while reasonable, simply does not apply.
In such a case it is very easy to see how it would be possible for the patient to choose treatment which is in fact against their best interest.
Also, as mentioned elsewhere in the thread the current regulatory/legal climate leaves a vanishingly small upside and massive downside to the doctor who would advise against treatment.
I'd think in the general case, yes. The patient is going to want it, even if it is only 25% effective. I'm not an expert on medical ethics, but in my mind if the patient has complete information and still wants the treatment, the doctor is not morally bankrupt in providing it. The doctor can certainly advise otherwise if they choose, of course.