Tim Spalding over on librarything has a great post called Percent who tag, as a follow up to his other post (When tags work and when they don't: Amazon and LibraryThing)
In it,he says:-
'Here's some data on that issue. I compared the number of books a LibraryThing member has with whether they tag or not. The later is defined as having at least one tag, so it over-represents taggers. But the trend is clear. The more you have to keep track of, the more you tag.'
I really like to tag stuff now (I really like the new feature in blogger for tagging).
The other thing I like about the article is it also engages the user with site (librarything I'm talking about). Some social cataloguing sites forget to engage there users, but not on librarything.