Robin wrote how being rational can harm you. Let's look at the other side: what significant benefits does rationality give?
The community here seems to agree that rationality is beneficial. Well, obviously people need common sense to survive, but does an additional dose of LessWrong-style rationality help us appreciably in our personal and communal endeavors?
Does LessWrong make us WIN?
(If we don't WIN, our evangelism rings a little hollow. Science didn't spread due to evangelism, science spread because it works. Art spreads because people love it. I want to hold my Art to this standard. Push-selling a solution while it's still inferior might be the locally optimal decision but it corrupts long-term, as many of us have seen in the IT industry. That's if the example of all religions and political movements isn't enough for you. Beware the Evangelism Death Spiral!)
We may claim internal benefits such as improved clarity of thought from each new blog insight. But religious people claim similar internal benefits that actually spill out into the measurable world, such as happiness and charitability. This fact gives us envy and we attempt to use our internal changes to group together for world-benefitting tasks. To my mind this looks like putting the cart before the horse: why compete with religion on its terms, don't we have utility functions of our own to satisfy?
No, feelings won't do. If feelings turn you on, do drugs or get religious. Rationalism needs to verifiably bring external benefit. Don't help me become pure from racism or somesuch. Help me WIN, and the world will beat a path to our door.
Okay, interpersonal relationships are out. Then the most obvious area where rationalism could help is business. And the most obvious community-beneficial application (riffing on some recent posts here) would be scientists banding together and making a profitable part-time business to fund their own research. I can see how many techniques taught here could help, e.g. PD cooperation techniques. If a "rationalism case study" of this sort ever gets launched, I for one will gladly offer my effort. Of course this is just one suggestion; everything's possible.
One thing's definite for me: rationalism needs to be grounded in real-world victories for each one of us. Otherwise what's the point?
This debate has already played out in attacking and defending Pragmatism.
A lot of the rubrics by which to judge whether or not rationalism wins or whether or not rationalism is an end in itself involve assigning meaning and value on a very abstract level. Eliezer's posts outline a reductionist, materialist standpoint with some strong beliefs about following the links of causality. Rationalism follows, but rationalism isn't going to prove itself true.
Deciding that rationalism is the best answer for your axiomatic belief system requires taking a metaphysical stand; I think that if you are looking for a definite metaphysical reason that you should practice rationalism, then you are interested in something that the practice of rationalism is not going to help much.
"Rationalism" as compared to what? Mysticism? Random decision-making? Of course rational behavior is going to be by far the best choice for achieving one's particular ends. I wasn't questioning the entire concept of rationalism, which clearly has been the driving force behind human progress for all of history. I was questioning how much making "tweaks" -- the kind discussed here and on OCB -- can do for us. Or have done for us. Put differently, is perseverating on rationality per se worth my time? Can anyone show that paying special attention to rationality has measurable results, controlling for IQ?