As a way of blowing off a little steam, a couple of Flickr employees launched a tag and pool for self-inflicted ugly photos, aptly named DoYourWorst.
Why am I pointing to this? Because while it may be a minority use, I maintain that tags as "social events/spaces" is an important use. In fact if I could figure out an objective means of classifying users tagging behavior as event or classification, the current high profile focus, an interesting study would be to look at the varied social tagging services, compare and contrast their features and see what style they encourage.
Even though Flickr and del.icio.us have very similar tag mechanisms there's something qualitatively different about how they're used. Something that's worth analyzing, quantifying, and understanding for future software designers.