Changing Tags on Blinklist, for example, can be very tedious: one has to enter each link and edit it. Why not change a group of tags at once?
What interests me is collaborative tagging. It is a grass root phenomenon and that's one of the main reasons why it is so cool. When unprofessional people make the tagging they enter mistakes, homonyms, synonyms, plurals, stop words etc.
I believe in Automatic Tagging as a tool that will be able to bring order to the current growing chaos of the Web. Automatic tagging as I grasp it will be very similar to grass root tagging. It will enter the same "noise": mistakes, homonyms, synonyms, plurals, stop words etc. Then the professional people will come with their librarian skills and correct the tagging: when they will see "search engine" and "search engines" they will decide to kill the plurals and let the singles live
But in order to unify these tags they will have to use the new feature of Changing Groups of Tags.