A "key" difference between a primary key and an object identifier is that the primary key is part of the table row -- it's part of the information content of the thing itself. An object identifier is metainformation, information about the object.

A reference to information in the row moves around wherever you move the thing. A reference to an object identifier has to be updated when you move the thing.

A query "Find me the page containing terms 'Hughw' and 'blog'" searches the information content of the thing itself. ('Hughw' and 'blog' are not primary keys of course, just ordinary "column information"). A URI to this page is like an object reference. If I move the page to another server, I have to update all links to it. And so do you.

Which do you think is a better technique for persisting references?