It may be rational. However, getting value from youtube, the blogosphere, facebook, myspace etc. is very closely interlinked to giving up a certain amount of direct control over your brand. Facebook is not a good place to put out press releases. Not every company should have a facebook account.
I think that this thread is an example of a growing issue in our history: PPC, SEO, Analytics, etc. These services are a good fit for almost all companies. A consultant can come in, build a site. Create an adwords campaign. Optimise & promote the site. The company/client gets sales, leads, bookings, etc..But the above process works. Almost for every market.
This is not the case with social media. Some businesses cannot get any value from facebook/twitter/youtube/myspace. Many of those who will get value will do so in different ways.
I'm not saying that there will be instances of success here. But they are (almost by definition) not everyone