It stops designers and developers from being "clever"
The user gets pages that are highly accessible, straight to the point and free from javascript malware.
Not at all, you can do lots of "clever" things in plaintext, like ASCII-art or aligning table columns with whitespace. This is much less accessible and adaptive compared to semantic HTML.
No protocol will prevent people doing fun and clever things or produce content which you think is useless.
No protocol will prevent people doing fun and clever things or produce content which you think is useless.