"You don’t have to sign up or explore any application for Thing 13. Instead, write a blog post on tagging. Does tagging belong in the library? Does it replace or complement subject headings? Should we encourage or discourage the practice?" (From NT23 Thing 13 task)
Tagging is fine when it makes sense. It's one of those things that could work well or just be a nuisance. There was one area of our website we offered tagging and no one used it, so it turned that feature off in favor of other ones. In delicious, it rules. I like the recommended tabs and for one of my projects, I came up with 10 recommended tabs for chat so the categories "made more sense". At least in that case, people seem to like the idea better than just bookmarking without intention.