Here are my views. First, they are one-to-many or a few-to-many format. Topics that require give and take, a conversation, would not work well on a 'blog. A message board, Wiki, phone or other method would be a better method if discussion was required. The tag boards and comments features are not enough to carry on a discussion. Everyone is a broadcaster on a Web log.
'Blog items are arranged in a chronological order. That is a valid method of accessing information in some circumstances. We have accession books (or used to) and chronologies in our collections. However, much information requires a different structure and presentation. Some 'blogs do have categories; The Shifted Librarian has posts by categories. That is not how we approach the site, that is an added tool. A text with an index is not an index. A book that arranged words by the date of usage would be interesting but not much use as a dictionary.
Web logs are on the Web. Reading a screen is not conducive to long passages. I'm currently working my way through The Nature of a Work by Richard P. Smiraglia. I would not read that as a Web page. It challenges me as a book, if I had the additional challenge of reading on a screen, it would be totally beyond me. The Web does provide the benefit of adding links and most 'blogs take advantage of that.
So 'blogs are useful to communicate short items presented in a chronological order, from one-to-many. News items jumps to mind or as a pointer to more in-depth information. They have the advantage of being available as an RSS feed or e-mail. That is user friendly. That is why I read them. I'll not give up going to meetings and conferences, reading book and magazines, watching TV, movies and DVD.