I think anything which promotes heterogeneity on the Internet promotes stability. Diversity in services, service providers, and separating the layers of the networking stack are all important.
The way that worms and viruses spread on the Internet is not that different from the way they spread in the real world, and the way you quarantine them is not that different, either.
I like the idea of separation of services. ISPs provide a pipe. Other vendors provide security. Other vendors provide email. When one party controls all the services, it's a 'synergy' for the company, but rarely for the consumer.
One of my bosses happened to be one of the early architects of some of the ways Internet providers work. He taught me how the cables connect, how the telecom providers work... I learned how to make my own Ethernet cables, all the way up to running a small business.
I grew up in Del Mar, Calif., north of San Diego. I got my first job the summer after eighth grade at a small Internet service provider.
With DNS, it's possible to control key components of Internet navigation. Google already controls search, they are quickly gaining market share to control the browser, and when you put in DNS, it becomes the trifecta of complete navigational control.
