I reposted this article in Discussion with a small addendum. Since your arguments are all really relevant, please repost them there if you have the time/inclination, so that people can still see and think about them. I didn't modify the article myself because 1) I'm lazy and need to get to class and 2) I'm hoping people will sign up before they read any arguments against it, because I do think that this is a way for people to put time towards charity that they otherwise wouldn't. But I respect and am grateful for your deconstruction of the mathematics of charity going on here.