Yes, I think this is a good idea. Is it so important that it needs to be in the software? Probably not. But I think it's a guideline that we should try to follow; certainly while editing.
The meta tags are very broad buckets. On its own, one of them is about as useful as a question title like "What do I do in this situation?" Even probably the most useful one as far as categorization, feature-request, doesn't give me a whole lot of information about the post on its own. People have lots of crazy proposals. I'm a busy guy. When you add flamingoes, then my interest gets piqued. The flamingoes are fine, why do we keep trying to change them...
So having other tags is important when the post is first made and active. It helps me decide whether I'm likely to want to engage with the question and answers.
It's also important later, when you're searching. If you're trying to find an old question that you remember, or even if you don't really know what you're looking for, that keyword being attached makes the search much easier. Especially if it's, e.g., the name of a piece of site functionality. More significantly, you can use it to go looking for old discussions or feature requests on that same topic. Like when I wanted to know why everybody decided the flamingoes should go to the left of the zoo entrance, which means their beautiful plumage gets bad lighting most of the day.
So this probably brings us to the issue of magnitude of usage... Well, like the classical radio station here in town likes to say, "All music was once new". Every tag started out tagging just one question. If it didn't get used for a whole lot of questions after that, I'm not sure why that's a big issue. If someone asks how to keep the flamingoes out of her inflatable pool, then flamingoes is a good tag to have on the question, even if not too many other people have trouble with their pink bird population. And unless the question itself is deemed unsuitable for the site and is deleted, it's hard to see why we'd get rid of a tag that is correctly marking that question with a descriptor.