Personally I like the subway clean and easy to disinfect. Many thanks to those who take the trouble to clean it. I do think graffiti breeds graffiti. And who can set themselves in judgment to decide what’s “good” and what’s “bad” graffiti? There’s very little that I’ve seen that’s not repetitive or derivative. (Ducks now.) If we have to have any then I’d say hoardings are fair game, as they’re (hopefully) not there long and if keeps people from tagging brickwork etc then I won’t object as long as it’s privately funded and cleaned.
Edit: AK8’s post about the Herne Hill children’s work appeared while I was writing my post. I do feel differently about little children’s work (though not on brickwork! - it should still be removable.) And I think it would have been much better if the Herne Hill children had got to design it too.