Landsburg's argument is sound, and I mostly follow it and occasionally try to sell others on it. But I can think of one exception, which is if the political power of an organization that you support depends on the number of members it has. So for example I pay membership dues to one organization that is not my main charity because I want them to be able to claim one more member.

And there is one place that I think Landsburg gets it plain wrong. He says*:

To please diverse stockholders, corporations tend to diversify their giving, often through the United Way. For individuals, by contrast, it really is quite impossible to justify that level of diversity. Surely among the hundreds of United Way recipients there are some you consider worthier than others. That means you can target your charity more effectively by bypassing the United Way.

But if you think that the United Way comes tolerably close to sharing your values, but you think that they have better information than you do about relative needs and competencies across different organizations, then it makes perfect sense to donate to them, doesn't it?

*http://www.slate.com/id/77619/