I agree, but what if a blog is setup by a bunch of different people - each running one category on the blog?
The difference with blogs and forums is that once the forum is started you can just leave it do it's job on it's own while you have a few moderators that you probably would get for free. With a blog you have to update it with new posts all the time. Although saying this, a blog is much easier to advertise as you have all them Blog rss directories, you have blog search on google, etc.