I believe that adding a single, relevant tag is a meaningful edit. If it helps someone in the future to find a question that matches their search, or favourites, then, yes, it's a good edit.
Note that tag editing is "so important" that for higher-rep users (10K tools) "inline" tag-editing is optimized and is very easy to do. I regularly add tags using the tag-editing shortcuts. It's obviously a meaningful action, so why prevent lower-rep users from doing it?
On the other hand, adding non-relevant tags is obviously bad....
I reviewed a number of those single-tag performance edits, and I approved them. If a person is willing to locate, fix, and suggest edits for them, then they deserve the +2 rep an approved edit will give them.
As it happens, while answering this meta-post, I also rejected a couple of tag-only edit suggestions where the calculator tag was added incorrectly... it can go both ways.