They're ivory tower ideals. The web has grown organically and it has the warts to prove it. It's useful to think about the spec and to work toward improvements

They're ivory tower ideals. The web has grown organically and it has the warts to prove it. It's useful to think about the spec and to work toward improvements, but if it's real world possible to have your services work well for mobile clients on m.domain.com and real world impractical to do so on your main domain (or any of the other examples regarding actions, languages, versioning, whatever)—then by all means, break the rules.