A copule of days ago I had lunch with a bunch of my co-workers at Jot (Scott, Michael, and Ryan) and I brought up the subject of tagging as a cop-out for search. Not surprisingly, most everyone disagreed with me to some extent. But as we talked it through, it became pretty clear that there was none of the "taxonomy vs folksonomy" debate in play.
Instead, people defended tagging from an interaction perspective. It seems that tagging feeds a need to feel organized in some sense. People at the table got a "solid" sense when they tagged a thing, regardless of its search utility. Better yet, people find tagging to be indistinguishable from social "trail leaving" (think del.icio.us), despite that their correlation is only an artifact of UI decisions.
It's just as simple for someone to click a "bookmark this" button as it is a "tag this" button, but somehow tagging has a more productive feeling for people. Tagging is a great way to entice users to provide other kinds of metadata, which is often significantly more useful.
Tagging is useful, but not for any systems reason. It's just a better UI experience. Who woulda thunk it?