Tagging, classifcation with another name?

After reading Clay Shirky's article on "Ontology is overrated", I feel slightly bemused by his discussion and ideas. On the one hand, he's right that we can't fit everything into the perfect system, because new information and ideas appear all the time, which then need adding into our system.  I'm the first to admit that Dewey is in desperate need of updating.  If we classified the world now, we would have a completely different system and hierarichal structure. However I'm none the wiser about his reasons for deciding that classification is wrong, because he seems to advocate tagging.

So, the first point to my mind is... tagging versus classification - aren't they the same thing? Maybe that is a very "Librarian" thing to say, but surely the idea is to enable people to find the items they want, and whatever helps them do that, is good.  Whether the information concerned is online or on a bookshelf.

If you are tagging a blog post or a web page, surely you're allowing other people to search and find it under a specific term, similar to classification, even if its not from a pre-defined hierarchy and is seen to be "free-form labelling".  Okay these labels may not be specific subject headings like Library of Congress or Dewey, but they are words refering to objects/items, even if the words are different depending on the person. 

To me tags and tag clouds are just another way of accessing online information, but far more visual. Using keywords and classification hasn't died, its just been modernised.