Imagine you had magical healing powers. Sitting quietly with someone and holding their hand you could restore them to health. While this would be a wonderful ability to have, it would also be a hard one: any time you spent on something other than healing people would mean unnecessary suffering. How could you justify a pleasant dinner with your family or a relaxing weekend at the beach when that meant more people living in pain?
But you already have these powers. Through the magic of effective charity you can donate money to help people right now. The tradeoff remains: time you give yourself when you could be working means money you don't earn which then can't go to help the people who would most benefit from it.
(I don't think this means you should try for complete selflessness; you need to balance your needs against others'. But the balance should probably be a lot further towards others' than it currently is.)
Update 2012-08-12: this is a response to hearing people offline saying that if they had magical "help other people" powers then they should spend lots of time using them, without having considered that they already have non-magical "help other people" powers.
I also posted this on my blog
This post is too low on interesting or useful new content, relative to reiteration of a standard ideological view about altruism. It would be different if the post described an interesting new argument, or a new more efficient means of helping people.
Really, this is just a particular form of "what if you had super-high productivity, orders of magnitude higher than almost anyone else on Earth." If I had these powers, and knew they couldn't be duplicated through research and study, I would sell them at high prices. To extract as much of the surplus as possible, I would use the "financial aid" price-discrimination system employed by elite universities, demanding tax returns and other information to determine ability to pay, and then extracting a large portion of that potential in exchange for healing.
If one has to "lay on hands" only briefly, the expected annual revenue (provided one didn't get kidnapped or imprisoned) would be in the trillions of dollars. Even if a healing took a couple of hours, revenue would be in the tens of billions (driven primarily by the super-rich).
In this situation I would probably work 70-100 hour work weeks, and use a budget of billions of dollars to make quality of life while working as high as feasible, e.g. healing on the beach while getting massages, eating gourmet meals, hearing reports of scientific research projects I had commissioned, and so forth. And then a majority of the revenues would go towards improving global prospects.
The opportunity to help others is is not taken advantage of by either altruists or egoists as much as it would fulfill their values. When people continue to act contrary to their values, it's good to continue remind them.