Blogs that turn into business

Zeldman asks: Should your blog have a business? And points out how design agencies like Coudal Partners and 37Signals (my faves from Chicago, or possibly anywhere) have, via their blogs, been able to attract enough people to buy their products that they've pretty much stopped being client services companies and turned into product development companies instead.

Meaning, if your blog attracts enough like-minded folk to buy product, why do you need to do client work?

Taken to another level, if my blog has attractive enough content to find a sizable paying audience, why do I need to work? I'm looking at you, kottke!