99.9% of blogs are crap, including mine. I write it mostly for myself and to polish my English (not my native tongue). I don't believe one should be looking for "advantages" when deciding to start a blog. I hate to see what blogs are turning into: they aren't personal anymore and look more like expanded resumes. Blogs used to be just that: a personal diary of what's going on. And silly things like "Rails Blog" or "Java Blog" just don't make sense: why would someone keep a blog like that - nothing is happening in their life besides Rails or Java?
In fact, let's be honest: a blog is just a buzzword. "Homepages" have been around forever, why rename them into something which isn't even a real English word. Before '03 corporate websites had a page called "News", now they call it a "blog" and make it look a little bit more personal.
After about 3 years of this "citizen journalism" I went back to slashdot, they provide more than enough of daily time wasting material. Real news are in books, never mind that they happened in the past, at least they're true and delivered to you professionally.
And finally, I don't trust "blogs". Even my modest page with 40 daily accidental readers attracts an email from a PR firm every once in a while, asking me to promote some BS they're getting paid for. I can only imagine how it's like for someone with 10K+ monthly uniques.. Most of these "bloggers" are cheap part-time journalists armed with inflated egos but lacking skills/time to check their facts and, sometimes, their spelling.