learning is clearly inappropriate when used as a meta tag. Meta tags describe the question, instead of the content of the question. You're correct that all questions are ideally about learning something from the answers, but using the tag for that reason is incorrect. Using it when the asker has a question about learning itself is appropriate.
As you mentioned, it's usually used to refer specifically to beginners learning to play. I've also seen it used for questions about learning new styles or techniques. I think this is useful. A piano teacher who joins the site might, for example, search
[piano] [learning] so that they can help some people out. An intermediate musician might search
[learning] so they can help out beginners with the basics, since they're not able to help experts.
In short, I see no problem with this tag if it's used appropriately. Updating the tag wiki to reflect its proper usage might be a good idea. And if a question is tagged wrong, please re-tag :)