This is mentioned on the blog:
Why Can’t You Have Just One Site?Why Can’t You Have Just One Site?
There is no (good) community that can form around “let’s just talk about everything and tag it”
I completely agree with Jeff on this. There's a more fundamental problem than just organization. An online society, just like its physical counterpart has a dominant culture. It shapes the society. At best, you can use tags to separate content but they can never separate the culture effectively and I believe SO, SF, SU, and MSO each require distinct social norms to be effective. Merging everything in one location, even if they are separated by tags, will degrade the overall quality of the sites.