I'd like to focus on the stakes of sharing academic blog posts and papers online.* In academic writing, blog posts aren't considered CV-worthy publications because they aren't peer reviewed and because they are perceived as informal. Rather, writing blog posts garners the author attention, connections, and possibly fame. What happens when an author wants to submit a blog post to an academic journal? As coeditor of the academic journal Transformative Works and Cultures, I helped create policies for submission of previously posted material. Full text readily available online usually means that the scholarship can't be submitted to a journal—any journal, not just TWC.
The reasons for this are mostly practical. The major reason has to do with clear copyright. Most academic journals, including TWC, have the author transfer the copyright to the journal. However, if the article has appeared online previously in substantially the same form, then who holds the copyright? Does the author hold it, because it appeared first in her blog? Sure, she can remove the content when she submits, but the Wayback Machine never forgets. Then there's reprint rights. Most journals charge $30 a pop for reprints. If someone can get the paper for free online, then the journal can't make money. Journals in particular want original content that can't be obtained elsewhere for free; exclusivity of targeted, vetted content is what drives readers to them. Then there's revision. Published papers go through a rigorous quality control process. The existence of an unedited version just means there's another version floating around that isn't identical to the final corrected version. Obviously the fully revised version ought to be the master copy.
Authors can take a few steps to make sure they can submit their papers to a peer-reviewed journal. If an academic conference wants to put the full text of the paper online (like Media in Transition does), then the online version of the paper ought to be completely rewritten for submission to a journal, and the cover letter to the editor of the academic journal ought to disclose the existence of the online version. The submitted paper needs to be substantially different than what appeared online. What does "substantially" mean? Usually the author has to make the case. In this sort of situation, I advocate focus: the focus of the conference paper ought to be the conference topic. When rewritten as a full-on paper for submission elsewhere, the topic may be broadened, and of course the paper will be longer. The main thesis may remain the same. The construction of the argument and the level of support will be different. If authors want to blog versions of the paper to get feedback, they ought to password-protect the post so the text won't show up on Internet searches. Authors could also post much shorter, probably hotlinked but otherwise academic-work-unsourced, versions of their ideas.
Some scholars feel strongly about open access and want to be able to hold onto copyright so they can disseminate their work as they like. That works fine for authors who are well published, famous, and/or have a relationship with a reputable press that will publish work when someone else holds the copyright, but alas, not everybody is Lawrence Lessig. Scholars who attempt to hold onto copyright when submitting to traditional outlets are more likely to be rejected simply because of the bureaucratic hassle. Such scholars are better off researching open access publication options. Most people, especially junior scholars, are hustling to get published and don't have the prestige or connections to get special exemptions.
Much of what I've written provides the rationale for why those in the publishing industry don't like to work with readily available online material: it creates confusing copyright issues, it affects their income stream, and it suborns the editorial and editing processes. But really a larger issue is at stake—an issue I don't have any answers for: what is the role of informal writings and musings within scholarly discourse? Is it part of the process of groping toward an idea, or perhaps toward a consensus with respondents? Is it a way to air questions? This was less of an issue when conference papers were delivered orally at academic meetings—there was no long-term physical record. How can we have a dynamic conversation in an era of persistence?
* Some disciplines, notably those in the scientific, technical, and medical market, make preprints available, or make available unedited versions of papers that are currently going through editing and production. Aside from certain instances of open peer review, as at MediaCommons, this is not (yet?) the case in the digital humanities.