As I write this http://staples.co.uk is reporting that it is down. There are probably quite a few people interested in shopping at one of their stores going elsewhere. If the site was actually down that would be fair enough. It isn't though. http://www.staples.co.uk works perfectly.
You quite often see 'server not found' warnings when visiting a site without www., for example my university, but to get through to the site and have it tell you the site is down is even worse. It demonstrates that whoever manages the server knows what they are doing but carelessly hasn't set things up correctly.
It doesn't matter which way round you have things but all traffic to www. should be redirected to the www free version or vice versa.
There are three main reasons why this is important:
- All your visitors actually reach your site
- No duplicate content issues in the search engines (this is assuming that both versions of the site actually work)
- Strengthens search engine positions by concentrating all incoming links on one site.
All it takes is a couple of lines in a .htaccess file if you're on an Apache server