You have a good point, but we're not using the web today in the document oriented way of the early 90's.
Sure, if someone will invent a way to use the semantic data in a way that could generate income or market value for the data owner/publisher, then the SemWeb will take off.
However in the current presented form, I don't see a strong incentive for any profit oriented agent to just put out their data in this way.
Depends on which data you're talking about and what the purpose of sharing it is. If it's a catalog of items for sale on an e-commerce site, then sharing that data widely - in order to make those items more discoverable - might be just what a company wants.
True. This means that the result will be just improved vertical searches of products which is kind of what price comparator sites already do pretty well these days.
"It doesn't take economics into consideration. most people/companies aren't that interested in sharing their documents in an anonymous way.
WWW is a web of documents. SemWeb is a graph of data.
More reading: http://www.w3.org/People/Berners-Lee/ http://www.w3.org/DesignIssues/