[masswerk]

A) Consider any sharing platforms where unrelated bodies coexist with distinct subdomains under a common root domain (e.g., Blogspot, Tumbler, etc) While "www" is probably a reserved name and mostly not of practical concern, "m" may be a practical issue.

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.)