Many webmasters use the www. subdomain without thinking. Sometimes you actually need to have the www. subdomain before you can even access a website. A good example of this is W3Schools. First without the subdomain: W3Schools with the www. subdomain. Now with the subdomain: W3Schools with the www. subdomain. W3Schools is a respectable site that I have learned a lot from, this however is a huge flaw and therefore they might lose a lot of visitors due to lack of support for their domain without the www. subdomain. (Note: W3Schools has fixed this problem after I originally wrote this article).
However, the real problem with the www. subdomain is the fact that people don't even know what it is. You probably know what WWW stands for (or now you do by hovering over it). What I mean is they think *for some reason* that www. is the protocol for accessing websites without even knowing what it is! They don't even begin to know what the HTTP is for and that there are other protocols like FTP and SMTP.
No-WWW makes a good point when they say,
By default, all popular Web browsers assume the HTTP protocol. In doing so, the software prepends the 'http://' onto the requested URL and automatically connect to the HTTP server on port 80. Why then do many servers require their websites to communicate through the www subdomain? Mail servers do not require you to send emails to recipient@mail.domain.com. Likewise, web servers should allow access to their pages though the main domain unless a particular subdomain is required.
There are even more disadvantages to www. for example I've met people who believe that www. and mail. are the only valid subdomains and http://securityresponse.symantec.com/ was probably made "hackers" (meaning crackers when translated into English) had made the website due to the subdomain, sad isn't it? Well that's what happens when we don't inform people.
Likewise from the Yes WWW website, ftp.domain.tld is not real. The protocol scheme is protocol://url/ . This means we should use http://domain.tld not www.domain.tld or atleast domain.tld. This also applies to ftp://domain.tld and not ftp.domain.tld, which would probably not work anyways and therefore it is a perfect example of how subdomains are not protocols.