It could totally work at the file-level too. Think of Book Designer files, that force bold onto every paragraph. No need for that; boom! they're all gone.
It would merely make the CSS apply styles to non-existent classes.
No harm, especially if a CSS validator comes into being, it might note unnecessary classes so you could clean it up.
Truthfully, you want as few classes and IDs as you can make work effectively and logically.
m a r