Crooked Timber has a great discussion of why academics do or don't blog. Brian Weatherson made some points that I think echo my feelings on the subject to some degree:

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.

From Crooked Timber: Academics and blogging
Referenced Tue Mar 23 2004 14:32:56 GMT-0700

Of course, this is true whether you're an academic or not. The larger question is do blogs matter academically, not just as a writer. Another comment says: "I've thought about it repeatedly and I keep coming up with the answer `I'd like to, but no.' I (very) occasionally contribute to a big blog. It's lots of fun. But the press of time - specifically [with respect to] tenure and promotion schedules - makes it impossible to commit that amount of time every day."

This last comment makes me sad because I feel its an abandonment of scholarship to a system that may or may not promote scholarship. Specifically, one of the chief goals of scholarship, particularly in the realm of science, is influence. Good academic work is likely as not to get accepted in a scholarly journal, which meets the needs of the tenure committee, but is only read by a handful of people. An entry on my blog is likely read by several thousand people. Which has more influence? That's not an easy question to answer.

Of course, 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.

One of my chief complaints about traditional scholarly journals is that they are mostly for profit enterprises. Even those run by non-profits frequently have to make money to support other activities. The result is a vast system with high overhead that cannot make scholarly work freely available without collapsing in on itself. So, when I publish something in a scholarly journal, it almost inevitably goes behind a pay curtain that makes the New York Times and Wall Street Journal seem positively freewheeling by comparison. This is in direct conflict with what scholarly publishing is supposed to be about.

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 judgment 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.

For a more lengthy treatment of this topic, see Where Do I List This on My CV? Considering the Values of Self-Published Web Sites by Steven D. Krause.