Considering a subdomain "trivial" is ridiculous... there's a difference between "www.example.com" and "example.com". Not only can they serve different sites, they can even have different DNS records! It seems that "m." is also considered a trivial subdomain. So when a user clicks a link to a "m.facebook.com" uri, they'll be confused why FB looks different when the browser reports it's on "facebook.com". I sincerely h…
>there's a difference between "www.example.com" and "example.com" Can you link to a site where these two are different?
B) Consider subdomains for test-purpose like "www.test.www.example.com" (now displayed as "test.example.com", which is actually not even the root of the specific subdomain).
C) Users unsure, if they are on the full-featured or a reduced mobile site, when "m" is hidden.
D) I may actually want to have a service agnostic default host at the root and subdomains for dedicated servers (like "www", "ftp", "mail", "stun", "voip", etc). Maybe this one just returns a short text message by design, if accessed on port 80. Not every domain is just about the WWW. (Edit: While we may assume that such a server would forward in practice, this may be assuming too much.)