The problem with that reasoning is that it discriminates against people whose physical condition incapacitates them to take actions affecting their lives. I am a rather healthy person, but if I decide that I've had it, I can off myself by a number means, from guns shot to hanging myself to jumping out of a window to cutting my wrists and to poisoning myself. I can do that regardless of what my significant others may think of it, regardless of how the army of priests and moralists view it, and regardless of what the law in all its majesty says about suicide. The only thing that matters is that I can and I am willing to do it - and if I do it, nothing else - gods, laws, morals, or any other mumbo jumbo - matters.
If I become physically incapacitated, however, the right to take my own life suddenly becomes subject to approval by priests, parents, doctors, judges, politicos and kindred bigger than life characters. In that situation, I can terminate my life only when these clowns -who otherwise would not matter - approve of it. That is discriminatory.
Therefore, the proper role of the state is to assure that the person's will is honored regardless of his/her physical condition. If there is doubt, I would rather err on the side on honoring the person's rights to have his/her will honored rather than trampling it. You would not advocate questioning an incapacitated person's other rights, say property rights, by subjecting it to inquisition by doctors, parents, priests, moralists and politicos, would you?
Wojtek