Tagging vs. indexing

The use of tags by readers has skyrocketed. According to the Pew Internet and American Life Project, January 2007, one third of U.S. Internet users (42 million Americans) had tagged some form of online content. 10 million Americans (7% of Internet users) are tagging content daily. (Gary Smith, Tagging, p. 18)

Here's a new study analyzing tagging practices. Is this "indexing by mob" or is it a valuable source of vocabulary?

PS. I really really recommend this book.
tagging