Over the years, I've seen a lot of articles advising writers to blog, often in a very rah-rah, not very helpful way. For instance, a blog does not take the place of a website, no matter how many people manage to publish articles at writing sites and magazines saying they do.
I just stumbled upon What Do Kidlit Literary Agents Think About Blogs? at Writing for Kids (While Raising Them) this morning. The general feeling seems to be that the agents don't feel that a blog is a necessity, but if they are considering signing a new writer, they will check out a blog if that writer has one.
What this suggests to me is that writer blogs are a lot less about marketing/promotion and a lot more about presenting yourself to your readers, some of whom may be publishing professionals.