I make websites for a living and no I don't make multiple sites for clients. When a client comes to me for a site, I make him ONE site. You still didn't answer why there isn't multiple yahoo.com, ronpaul2008.com, dailypaul.com ...
No, you are looking at two different things. To build a brand, you ened a central site.
But we are promoting an idea, not a brand, and more venues always help you promote the idea while they actually hurt you promoting the brand.
You never have a client ask for more than one website because a SINGLE client represents a brand, not an idea. But, for instance, for a portfolio, you are promoting both an idea and a brand (yourself). You usually have your "resume" site which consolidates the brand, and link to example sites to promote the idea.
It's the same concept. A single, scathing editorial in a paper promotes the author or the paper or the editor. But a slew of them in most major papers promote the idea behind the editorial.
You see what I'm saying?
And none of this is to say that we SHOULD have multiple sites, but its not as if there are NO advantages to having multiple sites. Nothing is as black and white as that.