There are several perspectives to look at this from.
Firstly though, I would say that Steve's blog is pretty much 90% about personal development. Even the post you mention about being a vegan is related to his own personal development, the reasons for it and how he improved his lifestyle.
Even if there are 10% of posts that were NOTHING to do with personal development I would still say that his blog is very focused.
Now then, the pro's and con's of having separate sites/blogs for each separate subject matter depends on the following in my opinion:
- How varied are the subjects? If the subjects are as varied as football, money, agriculture, fashion and monkeys then I would suggest separate blogs - but if we are talking, for instance, football, rugby, cricket, tennis - then one blog about 'sports' would be fine IMO.
- Are you interested in search engine rankings? This is probably the biggest make or break part of the decision. If you are (and I presume you are because you are after adsense clicks) then I would seriously consider just the one blog. The reason? One blog means that all of your nice fresh content will be concentrated in one place, and all of the links you receive will be concentrated in one place. In terms of SE rankings these are two very important factors. I know I would rather have 50 links pointing at one domain rather then 10 links pointing at 5 different domains (unless those five domains were in totally non-competitive areas).
- What is your overall purpose for the blog If it is just to make cash then I would suggest picking one good subject that you are passionate about and would happily write all day about - but that is just my opinion. From what I have heard, read and experienced you are probably better to either go for the route of having one or two very passionate, in-depth sites or going the total opposite of having hundred/thousands/hundreds of thousands of cheap sites you couldn't give a monkeys about!
In terms of the relevancy of the adsense ads on each page - I am not 100% an expert in this regard but I understand that the title, keywords and content of the page does have a fair bit of bearing on the ads delivered, but sometimes the overall theme of the site can effect it too.
I hope this helps and doesnt add too much to the confusion!
I noticed that Jill from HighRankings posts here so she will hopefully stop by and offer a FAR FAR more expert opinion than mine!
Have a great day