Blogs created to specifically promote books don't seem to get very far. Often times I have seen authors generate a flurry of early posts in the wake of a book release only to abandon the blog relatively quickly. Then they come back a few months later, with a promise to get back to blogging, but this is often more of a last gasp than a true comeback. As Tom aptly points out, while these attempts might not hurt they do not seem to do anything to help a book that is already struggling to find an audience.
And, might I add, it's really annoying for those of us who actually wanted the writer to update the damned blog.
Besides, it's not like there's an instant audience. If your book's not doing well, your blog won't magically make things better. The intertoobz are vast, and there are many blogs: you're just another face in the crowd. Jumping up and down screaming "I published a book! Buy my book!" won't do jack diddly shit for you. It takes time and effort to build an audience for your blog, just like it takes time and effort to build an audience for your books. And your blog, like your book, has to offer a quality product that people want to spend time with if it's going to succeed.
Blogs and books are two different animals. And they both make outrageous demands on your time. If you don't have the time or the interest to split your focus, don't split your focus. You'll end up doing both your books and your blog a disservice if you're not actually interested in being a blogger. Stick with Facebook instead.
(Of course, Facebook has its own hazards. Anyone who's gone on to post a simple update and ended up sucked into about 5,000 Facebook games knows what I mean. Use at your own risk.)
Now, as for using your existing blog for shameless self promotion.... do it. Let me use Brian Switek, author of the above-linked post, as an example: I learned he was writing his book because of some modest little posts he wrote early on. He kept us updated on the process as he wrote, edited, found a publisher, wrote and edited some more, and now can triumphantly announce that, should all go well, we're going to have the opportunity to hold his book in our very own hands sometime this fall. If he'd kept his mouth shut, we wouldn't have anything to look forward to other than his wise and wonderful posts. And while his posts are wise and wonderful, it's even more wonderful to know that they're not the only written words we're getting from him.
Note also that Brian hasn't fallen down on his blogging duties. His posts have continued being wise and wonderful even when he's overwhelmed by work.
So, upshot: if you're going to blog, do it for more than promotional purposes, and give your readers something of value. But if you're already blogging, don't be afraid of a little shameless self promotion.