www. is just a subdomain in the old days it was needed to identify the URL as a web resource but it really isn't any more... Plus it makes the URL shorter
Lol exsactly the SEO urls already makes it longer, why would you want to add ANOTHER 4chars
Of course on your site you can choose to use www. with your xenForo install if you want though I recommend that you make it so that it only works with www. or only works without
www. is a relict from the dark ages of the internet. I never use it anymore. And the few websites who are reachable with www and not without, incur my wrath. My wrath consists of saying "fail" out loud.