I just read a bit that reminded me of Dave objecting to Dawkins' view of religion, but not quite being able to say why.
I can't be bothered to type the whole passage in this time, so I'll summarize wildly, You really should buy the book and read it (this is chapter 7, Age of Chiefdoms).
Dawkins compares memes to viruses, not just to genes. There aren't many prolific memes that are like viruses (Wright suggests the meme for injecting oneself with heroin). He says the brian has got quite good at rejecting memes that are bad for oneself, and retaining those that do some good (or are at worst benign).
Memes that manage to get through the various individual and cultural selection mechanisms are often those with non-zero-sum value to them, and most religions fit this mould (with the obvious exceptions of those like Heaven's Gate). Most religions are averse to theft and free-rider behaviour, thereby enhancing the trend to non-zero-sum exchanges.