I consider runes spam. However, there might be a valid use case. Could there be value in storing sensitive information you want to selectively reveal to the bitcoin network? You can write encrypted data in transactions but then it's obvious you're sending messages and you (or the message) can become a target.

There's a concept called Steganography [1] people have been using for centuries to share secret messages.

Note that encryption isn't equal to secrecy. If you send an encrypted message it is obvious you have done so, but if you hide that encrypted message in a JPEG then you raise fewer red flags.

For this reason, we should be very careful when discussing how to combat spam.