When should one use a 'www' subdomain?

Question

When browsing through the internet for the last few years, I\'m seeing more and more pages getting rid of the \'www\' subdomain.

Are there any good reasons to use or

Answer 1

Take it from a domainer, Use both the www.domainname.com and the normal domainname.com otherwise you are just throwing your traffic away to the browers search engine (DNS Error)

Actually it is amazing how many domains out there, especially amongst the top 100, correctly resolve for www.domainname.com but not domainname.com