I think blog "rules" are limiting. And, it matters what your goal is. Some people want Digg front page status - but those posts are all very similar: stupid humor or really long lists of technology-related info. It's geek humor. Cool. But not my style. And I'm guessing not yours. 

I have to say that I think "useful" is a really broad term. Useful for some people would mean long lists of tips and tricks and hacks. While, useful, to me, is a blog I can relate to and feel inspired by. 

I don't have a husband, a dog, children, or any faith in Christianity, but your blog speaks to me. And that's, in my opinion, the most useful you can strive to be. Because, while these useful blogs that the article talks about may be giving us all these tips, it's not being useful for us in connecting with others. And, that's the cornerstone of humanity, not tips and hacks for getting things done or getting a job or whatever else. Not that there is anything wrong with those posts, because there's not. 

But, limiting the usefulness of blogging to tips and tricks is narrowing. And it stops the flow of thought. And, I say, write from the heart and the right people will think your blog is useful. Because it is.