Focus on adding meaningful tags rather than placeholders.
the wiki says that any image should be tagged copyright_request and artist_request if there are no proper tags
It wasn't really discussed, it seems like someone used a translation_request guideline (that you shouldn't remove it until the post is completely translated) as the definition for all of them.
*_request tags should be added when you, personally, want to know something - it's a request.
They're also useful for "only some of them are tagged" situations, as mentioned above.