The fact that someone would be willing to forgo a decent diet in order to build a monument to his god does not mean that his claim on others for aid in his project has the same strength as a claim for aid in obtaining enough to eat. (Scanlon 1975)
Consider next Desire-Fulfilment Theories. The simplest is the Unrestricted Theory. This claims that what is best for someone is what would best fulfil all of his desires, throughout his life. Suppose that I meet a stranger who has what is believed to be a fatal disease. My sympathy is aroused, and I strongly want this stranger to be cured. Much later, when I have forgotten our meeting, the stranger is cured. On the Unrestricted Desire-Fulfilment Theory, this event is good for me, and makes my life go better. This is not plausible. We should reject this theory. (Parfit 1984)
If I have a bad headache, anyone has a reason to want it to stop. But if I badly want to climb to the top of Mount Kilimanjaro, not everyone has a reason to want me to succeed. I have a reason to try to get to the top, and it may be much stronger than my reason for wanting a headache to go away, but other people have very little reason, if any, to care whether I climb the mountain or not. Or suppose I want to become a pianist. Then I have a reason to practice, but other people have little or no reason to care if I practice or not. (Nagel 1986)
For example many Boston residents desperately wanted the Red Sox to win the World Series in 2004. Their happiness when the Red Sox won gave others – even Yankees fans – some reason to judge it to be good for them that their preferences were satisfied. But, as Nagel maintains, the mere preferences of Red Sox fans, as opposed to their happiness or unhappiness, is of no moral importance to others. (Hausman 2012)
Us not wanting people to do things with our body without our knowledge is indeed a different argument, one which seems to show that at least some preferences matter ethically. But preference utilitarianism is usually the view that only preferences matter, perhaps even all preferences.
Regarding Parfit's case, is this not the same as me being unconscious while my body is manipulated? In both cases we do not seem to currently hold a preference. In one because he forgot about it, in the other because I'm unconscious.
But even suppose Parfit did not forget about the stranger. Why would it be good for Parfit that the stranger is cured, without his knowledge? To me it does not seem to be good for him. And wouldn't such a view have the unfair consequence that it is much less important to cure a lonely person about whom no other people care than a popular person about whom lots of people care, even if those are not informed about the cured illness?