I'm not convinced that there's any reason they should be intersubjectively true (socially beneficial) either.

It seems clear to me that memes are socially beneficial in the sense that we're much better off with the memes that we actually have (including traditional moralities, laws, etc.) than no memes, or a set of random memes. And also that it would be quite hard to find a set of memes that would do as well, if we were to start over from scratch. I'm not quite sure how to explain this, or answer your other questions, but perhaps Nick has given these issues more thought. He recently reposted the essay to his blog, so commenting there might be a good way to draw his attention.