I have 8002 tags which is not an ideal situation by any means. Periodically I take a stab at weeding out a few hundred (I had 11,000+ at one time), all from within books or from metadata downloads.
Still, like kiwidude, I was disconcerted to have my tags overwritten, and be left with a single tag usually genre.
many (perhaps most) of my tags are repetitive and useless but I still get a lot of use from the tag function.
Recently I read a steampunk novel and was looking for more. I only had 1 book tagged steampunk and that was the one I had read, so I clicked on Victorian and control clicked on urban fantasy and voila I had a few more
IMO too many tags are better than not enough.
Helen