Some additional related points:
1) Joao Fabiano looked recently into acceptance likelihood for papers in the top 5 philosophical journals. It seems that 3-5% is a reasonable range. It is very hard to publish philosophy papers. It seems to be slightly harder to publish in the top philosophy journals than in Nature, Behavioral and Brain Sciences, or Science magazine, and this is after the filter of 6 positions available for 300 candidates that selects PHD candidates in philosophy (harder than Harvard medicine or economics).
2) Bostrom arguably became very influential before Superintelligence within academia and the Lesswrong world. I have frequently found academics in the fields of Philosophy of mind, ethics, bioethics, physics, and even philosophy of language who knew his name and had some idea of who he was. Bostrom is a very prolific academic in terms of paper publication, and notably he pays attention to having an open website with his ideas available to the public. I believe his success prior to Superintelligence is explained, besides sheer brilliance, by actually displaying his ideas online as well as creating the World Transhumanist Organization and using techniques like talking about normal seeming topics (the matrix, cars on the other lane) in papers which are also strategies that helped make David Chalmers, another brilliant academic, become more prominent.
3) Joao Fabiano's inquire also found that David Lewis published, singlehandedly, 6.3% of top five journal publications. All women together published 3.6% (including Ruth Millikan, very prolific author). To me this indicates that the power law for publications is incredibly strong, with only extremely brilliant people like Lewis making the most substantial dents. The field is also male-skewed in an awkward way.
4) For people who consider themselves intellectual potentials and intend to continue in academia, my suggestion is to create a table of contents for a book, and instead of going ahead and writing the chapters, find the closest equivalent of some chapter that could become a paper, and try to write a paper about that. If you get accepted, this develops your career, and allows you to be one of the stand-outs like Ord, MacAskill and Bostrom who will end up working in the top universities. If you continue to be systematically rejected, you can still get around by publishing books and being influential in the way say Sam Harris or Richard Dawkins became influential. Since I find this to be the optimal strategy I'm aware of at the moment, it is the one I'm taking.
5) One behavior I found dangerous in the past is to "save your secret amazing idea from the world" protecting it by not testing it against other minds or putting it out for publication. Idea theft may be common in academia, but the response to that should be to simply have another better idea and carry on the work. Most ideas are too complex for non-authors to be able to steal. I think great value can come from publishing your ideas on Lesswrong (Stuart Armstrong does this) or Effective-altruism.com before transforming them into a paper, and eventually a book chapter. This is how I've been reasoning lately. (To put my money where my mouth is: if anyone wants to take a look at the table of contents of my academic Altruism: past, present, propagation book by the way I'm open to that, message me privately) I welcome any criticism of that blog--> Paper --> Chapter strategy.
6) Most of what I said applies to philosophy and the humanities, and I'd be interested to know if people who published in STEM fields like Paul Christiano, Nate Soares, Benja Fallenstein, Scott Aaronson and Tegmark think this is a valuable alternative there as well.

I find a lot of value in academic papers, especially in STEM fields, and am going to spend some time outlining my defense for writing them. That being said, I'm not necessarily looking at them as a "discussion forum for EAs". I think there are many reasons why academic papers can be useful to Effective Altruism even if they're not directly geared towards promoting EA-type ideas (though they certainly can be). Specifically, I think they're hugely important in research, and not just for helping secure a research-type job.
Academic papers are, in general, more mathematically rigorous than books. This is not to say that books can never be as rigorous as academic papers, but the overall trend is that books tend to summarize information or lay out information in layman's terms, whereas academic papers lay out detailed information in very data-heavy terms. Thus, academic papers may or may not be good for discourse depending on the audience's interests. Take Kahneman's books on cognitive psychology/behavioural economics--although his books are probably more popular than his papers (by people-at-large), his books reference his papers. It would be folly to try to publish his experimental results as a book (where would you store the results and supplemental materials)?
Academic papers also take less time/energy to write than books, especially for incremental research. For instance, if you're trying to demonstrate the efficacy of a particular anti-malarial drug, it's much less time consuming to publish this in a paper than it is to write a book about it.
Most importantly, academic papers are peer-reviewed. You can trust that papers that have come out in reputable journals have the support of experts in the same field. You really can't do the same for books. Take the large amount of books that exist in support of intelligent design. None of these would hold a dime in any reputable genetics or biology journal. In many cases, the reputability of a book is often dependent on the number of citations it has to journal papers or other material. (Again, this is not to say that books can never be reputable--you just have less confidence about whether one is or not.)
I'm more curious about specific cases of where these book-vs-journal-paper questions arise.