Here The Search Guy claims that inconsistency in assigning keywords is the problem with tagging, and I would say that that same inconsistency is the whole reason why tags are useful in some instances. Because if I'm looking for something that I've already found, I want to be able to search for it in the context that I remember it, and not in some arbitrary other context that someone else imagined. It's not a replacement for subject headings in most cases, but is another way of keeping information.

"On a tactical level, this means that web sites that focus on improving their content, updating more frequently, recruiting users through community, etc. are missing half the picture--the half that says that if you issue APIs for your site's products and services, allow remixes, encourage--no, help--users tag your data--and RSS-ify everything--you'll be far ahead of the game and grow links--and audience--like crazy because your discoverability will soar. In other words, you need to not only improve your destination, you need to move off it."