Using URLs without www., I noticed that people adding links to your site prefix your URL out of habit, or maybe to make a "better" URL and do the site owner a "favour".

Now stupid search engines see two sites with each one counting for half of the incoming links, instead of one higher ranked site counting for all of the links.

As a consequence: when enabling both (or even more) URL variants, you should at least redirect the user (and the spiders!) to only one distinct host name.

Jens.