Is (was) SEO the only reason to force
the www. on domain names?
Are there any other reasons why an
authoritative domain name/convention
would be desirable?
Run Google's Page Speed - if you are serving cookies under the root domain (i.e. example.com) you will find that those cookies (per RFC 2109) are being sent as part of every HTTP request, including requests on static content served under subdomains.
Instructing clients to send spurious cookie data on every request is a sub-optimal solution - if you want to see the fastest possible turnaround on client requests, it pays to restrict cookie usage to a subdomain (i.e. "www.example.com") and put your static content on a separate subdomain.
See the Page Speed Web Performance Best Practices document for further suggestions in this vein.