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.