The thing is, www prefix was never a requirement. It came about as a convention to very specifically indicate that www.example.com = web server, and ftp.example.com = ftp server, etc. There's no real reason to do it, other than convention from the early days. The CNAME restriction is really the only technical gotcha, and this is slowly becoming a non-issue with DNS services like Route 53 that now allow CNAME on roots.