

Www. is deprecated - caustic
http://no-www.org/

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Tichy
Unfortunately on Google App Engine you can not use primary domains (or
whatever they are called). So www seems like an obvious choice for apps there.

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johannchiang
Keeping www allows hosting static content in cookieless subdomain (without
getting new domains) which speeds up page loading.

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momokatte
Using www.* gives you more options for load-balancing and distributing your
website too, since an A record can only resolve to an IP address.

I recently learned that the hard way when the Amazon Elastic Load Balancer IP
address we were pointing our A record at abruptly went out of service.
Serendipitously, we had already been preferring www.* for some arbitrary
reasons so it was an easy fix. And it turns out we're reaping the benefit of
what Johann describes too, as we are also serving a large number of images
from a separate subdomain.

