Maybe SEO for Google, ie if Google said "we'll improve your rank dramatically if you included semantic data". But even then - it would be an enormous amount of work to update all (or the majority of web sites).
There are over a trillion web sites. Why did people put in so much work? Because they had something that they wanted to be found/seen/known by someone else.
The semantic web, in the form that it will come in, won't be nearly that much work. Instead of each business creating a web site, they'll just fill out the data of what makes them what they are and what makes them unique.
There are a trillion textual web sites, nearly all created in the last 15 years, most of which don't need to exist. My prediction: Very soon, they won't.
Edit: I'm not talking about current initiatives like LinkedData. If the SW would only come like that, I would agree with you.
Just another node on a semantic graph is not what anybody wants to be, otherwise the web would be populated with HTML 1.0 pages. It's not like making one of those is much harder than filling out a semantic form.
> most of which don't need to exist.
Who says that? I think my page on mauve socks should certainly exist. There's an awful lot of stuff like that on the web. And the SW is no more useful for finding that than search is.
Their official jargon is "Rich Snippets", and they have a couple posts on it at http://googlewebmastercentral.blogspot.com/2009/05/introduci... and http://googlewebmastercentral.blogspot.com/2009/10/help-us-m... .
And, of course, the "semantic web" will be able to disregard bad data (published out of malice, incompetence or bitrot) because it ... hmm ... because ... hmm, er ... ah, well. They must have it covered.
(I'm somewhat reminded of this: http://en.wikipedia.org/wiki/Expert_system , only with the maintenance problems squared or cubed.)
P.S. The RethinkDB guys (Slava, etc.) are chasing an interesting problem (DBs on SSD). I submitted a link to their blog: http://news.ycombinator.com/item?id=1337391