A relationship between two rationalists can be much happier and freer of drama. If Eliezer's example isn't clear enough, here's another one.

"I'm worried about X."

Non-rationalist: "I've told you a million times, that's not gonna happen! Why can't you trust me?"

Rationalist: "Ok, let's go to Wikipedia, get some stats, and do the expected value calculation. Let me show you how unlikely this is."

Which conversation ends in a fight? Which conversation ends in both people actually feeling more at ease?

There are female memes to the effect "Men are endearing fools," and male memes to the effect "Women are beautiful fools." But a fool eventually gets frustrating. It is an incredible relief to meet someone who isn't foolish. "Whoa... you mean you can embrace an idea without being an uncritical fanatic? You mean you can actually make allowances for overconfidence bias, instead of taking reckless gambles? You can listen to the content of what I'm saying instead of the applause lights?" Having a rationalist partner means never having to say "Oh, you wouldn't understand."

Also, on cultishness: I saw an ad the other day for a new book on how to start a green activist organization. How to attract members, get speaking engagements, raise money, build momentum, etc. My first reaction was "Oh, that's nice; I'm sure that book would be handy for environmentalists." Then I thought "If we did half the stuff that tree-hugging college kids do, we'd call it Dark Arts and we'd be terrified of turning into a cult."