Someone says you should send all your money to Africa, because this will result in more human lives.

To which I would respond "Okay, but is that necessarily a good thing?"

Here you are, spending a part of your precious life reading Less Wrong. If you spend 10% of your life on the Web, you are saying that that activity is worth at least 1/10th of a life, and that lives with no access to the Web are worth less than lives with access. If you value rationality, then lives lived rationally are more valuable than lives lived irrationally. If you think something has a value, you have to give it the same value in every equation. Not doing so is immoral. You can't use different value scales for everyday and moral reasoning.

I think many of the reasons I disagree with your post as a whole have kernels in this paragraph.

If I spend 10% of my life on the web, it doesn't necessarily mean I value going on the web at least by 1/10th of my life. I think the truth is closer to "10% of my time awake, I was bored and happened to have a computer with internet access nearby". If you offered someone the deal "we will extend your life by 10%, but you may never access the web again"... well, I wouldn't accept the deal, but I'm sure there exists people who would; people who have spent 10% of their lives on the web.

And just because I spend 10% of my time on the web doesn't mean I value lives that go on the web more than lives which don't. This argument is as much of a non-sequitur as saying "People who spend 10% of the their time masturbating value lives involving masturbation more than lives that don't".

And I can easily give lives different values, while remaining self consistent and rational. Perhaps I'm selfish and I value my life more than any one else's lives. If the king had gave me the choice of either I die, or one other person dies, then I'd choose for the other person to die. If it was me against two other people, I'd probably still choose myself, though I'd feel guiltier about it. When it becomes ten is when I really start to hesitate, and when it becomes a million, I guess I really have to give up and allow myself to die.

Unless, of course, the world is already suffering through an overpopulation crisis, in which case by allowing those million to die, I could claim to be acting in the greater good of the world.