Eliezer wrote, "Really, I suspect that what's going on here has less to do with the motivating power of eternal damnation, and a lot more to do with the motivating power of physically meeting other people who share your cause."

I think this observation strikes very close to the heart of the matter. People will tell you they attend Catholic mass, for example, for any number of reasons, most of which are probably not available to introspection, but which actually relate to our functioning as social animals. People are motivated to meet other people, and church attendance is one of the few remaining outlets for this tendency. Whether the rationale for congregating is to uphold some deep cause is almost beside the point for the majority. Certainly there will be some for whom some cause is salient and pressing; they are the visible and the vocal, not the representative.

It is perhaps tempting to offer up Catholicism as a proxy for Christianity, and Christianity as a proxy for religion. Bear in mind, though, that Western and Eastern Christianity do not even agree on when to celebrate their holiest day, Easter. Then there are some rather yawning doctrinal disparities between Protestant sects and Roman Catholicism, and even then within itself. There is a growing movement among Catholics to have the mass recited to them in Latin. A prototypical rationalist might call this practice an example of willful obscurantism and worshipping ignorance, but that would be incorrect. It seems unlikely that the point is to comprehend the words of a dead language, but rather to use the setting as a means of priming a type of experience that William James was talking about.

In addition to Yvain's catalogue of Catholics being communal, hierarchical, and official, one could add socially reinforcing. If your co-religionist is giving 10% of his income to support church activities, and you know this is true (it's easy to find out), then there is social pressure not to be a slouch when it comes to tithing--a phenomenon of competition in terms of signaling commitment to cooperative behavior. Would rationalists would ever do that?