Why do people join it? Why does someone self-identify as a rationalist?
For the most part people are rational within the framework they have that allows them to get by in the world. Sure it's important to question beliefs to further the state of science, etc, but why is is it critical that any idea one holds be proven to the best of our abilities? I don't understand why belief in god has to imply there is anything magic or supernatural; it just might be something we don't understand. Certainly it seems possible that the universe exists within frameworks whose workings can never be fully understood from our current physical position. The rationalist view seems to be to let the astrophysicists do their work, but don't you dare believe in something that is outside the realm of what they will eventually discover.
I understand religion has done a lot of awful things in the past, and so it deserves to be torn down as an institution, but most bad things are propagated by extremely rational actors out of greed and self-interest. I don't see the harm in people believing in astrology or not being atheists, I'd rather see the rationalists take their arguments against the people who are actually doing major harm in the world (eg. profiting from massive environmental destruction).