Why might academics blog? Crooked Timber's discussion on this issue includes this from Brian Weatherson:
First, having to get my thoughts into a state where it's not embarrassing to have other people read them is a real spur to clarify what I'm doing and sort out bugs. Second, when I go to write the paper, I've got first drafts of some of the trickier sections already written, so I can cut-and-paste them in and start editing. Third, whenever I have an idea that isn't going to go anywhere, my readers tell me about it fairly quickly. That can save a lot of time, because there's nothing so bad for work output as spending lots of resources on a dead-end project. I've been saved from a few disasters this way.
In a post on his own blog, Phil Windley comments that 'my blog is missing something that academic journals have in spades: the trust of tenure committees. So while I can't say for certain which has more influence on scholarship, I can tell you with certainty which has more influence on my career'. He concludes:
Publishing in journals used to be the best means of promoting one's ideas and exerting influence in a field of thought; it has become one of the worst. Technology has moved beyond the high overhead of print publishing, but since the judgement of peers, in a very narrowly defined sense, has become the way we judge the quality of a scholar's output, the old system continues. I'm not thinking I can change that system, but it doesn't mean, as someone who values scholarship, that I shouldn't do that which is best even as I do that which is required.

