Hi @jefito thanks for sharing your thoughts, these are undoubtedly valid arguments.
From your description I realize that I use Evernote a bit differently. First of all I use the GTD method with 1-inbox, 2-next-action, 3-done [...] notebooks setup. This means I never use tags to indicate what actions "todo" etc but rather place the notes in different notebooks to keep track.
Also I practically never delete any notes as they are all eventually archived in the notebooks 3-done or 9-archive. This way once I used a tag it will live forever since I never remove tags from notes and rarely deletes any notes.
So for me the option to auto remove unused tags would be practical. But it should definitely be a checkbox in the preferences, since it would not suit everyone.
Thanks for the tip about the "Delete all unused tags" in the help menu, I read about this somewhere else, but I am also a little bit scared of using it.. ^^