The only interests that matter are the interests of the person who executes the document.
Those interests change over time.
The way we perceive medical care when we are healthy is very, very different than the way we perceive it when we are sick.
When these titans clash, it's best to have one's affairs in impeccable order. It's also best to leave one's power of attorney in the hands of someone absolutely trustworthy with one's life -- but I'm not telling you anything you don't know.
So, again, what's wrong with having documents drawn up, placing the affairs in order, beforehand? You seem to imply, Noble, that it's not a good idea for the patient.
It should tell you something that the main pushers of "living wills" today are the government, and HMOs.