Funny story. Just today I read the dissertation of a Rutgers philosophy PhD student (Rutgers is the #2 philosophy department worldwide according to the standard ranking ) on strategies for action in the face of normative uncertainty. It explores at great length the idea of instrumentally valuable strategies that boost our ability to attain a wide range of goals, and in particular the goals that we would seek under various idealizations.

Its introduction cites three inspirations leading to the work: John Stuart Mill's "On Liberty," John Rawls' "Theory of Justice," and Eliezer Yudkowsky's "Creating Friendly AI" (2001), discussed at greater length than the others.

EDIT: Because of the OP, I should note that I do favor SIAI publishing in mainstream venues.

I agree with previous comments about publishing in journals being an important status issue, but I think there is other value as well which is being ignored. For all of its annoyances and flaws, one good thing about peer review is that it really makes your paper better. When you submit a pretty good paper to a journal and get back the "revise and resubmit" along with the detailed list of criticisms and suggestions, then by the time the paper actually makes it into the journal, chances are that it will have become a really good paper.

But to return to the issue of papers being taken more seriously when published in a journal, I think that this view is actually quite justified. For researchers who are not already very knowledgeable in the precise area that is the topic for a given paper, whether or not the paper has withstood peer review is a very useful heuristic cue toward how much weight you should place on it. Basically, peer review keeps the author honest. An author posting a paper on his website can say pretty much whatever he wants. One of the purposes of peer reviewers is to make sure that the author isn't making unreasonable claims, mischaracterizing theoretical positions, "reviewing" the relevant previous literature in a grossly selective way, etc. Like I said, if someone is already very familiar with the area, then they can evaluate these aspects of the paper for themselves. But if you'd like to communicate your position to a wider academic audience, peer review can help carry your paper a longer way.

I think you're underestimating the status arguments in favour of publishing in journals. Status games are really really really influential in our world. We ignore them at our peril.

Articles in mainstream journals take a relatively large amount of time, money, and expertise to produce.

As do those articles in the first place. What is the incremental cost of making them fit for mainstream journals?

It is easier to simply collaborate with (and greatly influence) established mainstream academics who have already jumped through mainstream academia's many hoops (as Carl Shulman has been doing, for example).

Shulman is credited as co-author, IIRC. So wouldn't this still be SIAI publishing in mainstream journals?

I don't see why 1. is a reason not to publish. Presumably Bostrom's pre-prints went on to be published.

Data point on why we should publish, The Singularity as Religion mentions:

The singularity is based on science rather than superstition. My reply is that much of what I read about the singularity goes so far beyond any scientific data or present technical achievement that it looks very unscientific. Perhaps someday there will be an artificial general intelligence that far outperforms humans. But plenty of scientists and engineers seem highly skeptical about the grandiose claims of singularitarians. Are any of the claims of the singularitarians empirically testable? Verifiable or falsifiable? Only in some indefinite future. This is what John Hick called eschatological verification. But that’s not science at all. An interesting point here is that many singularitarians don’t seem to be interested in scientific research – such as writing papers for peer-reviewed journals. There is no such thing as the singularitarian research program in any standard academic or commercial sense. It looks like what Feynman called “cargo cult science”. And singularity activists have their own version of Pascal’s Wager. The singularity is so overwhelmingly transformative that even if it has a teeny-tiny chance of happening, the reward or punishment for us will be extremely great. It’s so easy to see! You just have to write out an expected utility equation.

Emphasis mine.

Is this a place-holder for an anti-publishing post that will be as well-argued as your pro-publishing post? You may have updated hugely but you haven't given us the tools to with your four-point summary.