Must a Blog have a focus?

I read somewhere that a "blog" must have focus to be successful. It must have a niche. Similar to marketing a product. My argument was, stuff that, my niche is anything that comes to my mind. Rambling stuff. In essence what has happened is that my the posts have gravitated towards five major areas, popular mythology, Nautical stuff, origins of sayings , no brain visual stuff and a tad of ramblings. I have the opinion that if it takes longer to read a post than you can hold your breath, 90% of the readers won't read it, unless it is very, very good, or you are talking about something very specific. There are a bunch of things I read about, but will not write about the same stuff. Personal relationships I enjoy if it it done without crying in the soup, Christina for example, and American politics, Carol. I get a better understanding of international politics from blogs than any CNN SKY BBC or EuroNews can do. Anyhow, I am digressing as usual.
I also saw a post somewhere that classified blogs as Commercial, People who write about themselves and people who write about things. Some write for themselves and don't care if it is read by anybody (a kind of therapy) and those who want to be read.
The heading of this post has the word "focus" in it, and I have lost focus already.
Anyhow, while scratching around on Amatomu this morning I had a look at the stats, which I don't really give much attention to, and the most popular pages read over the last week are depicted graphically. This points to a definate niche. Irrespective of what I post, the "reads" have created a niche. Call it a Readers Niche for the want of a better description.