Well leaving technical arguments aside for the moment, the overwhelming majority of web users type in addresses as www.site.tld, not just site.tld. Having a redirect will handle this circumstance, and is the most user friendly and possibly the most search engine friendly as well, though they can likely handle either easily.
Having it simply fail on the other hand is, in my opinion, a poor practice and most unfriendly to the web user trying to reach your site. While this may not matter for a low traffic and/or personal site, for any site that wishes to attract the average web user it can lead to lost visitors, or customers if the site is revenue generating. Even if it doesn't, it will likely lead to a certain level of frustration with using the site. Whether you choose to redirect the www to the base domain or not (or vice versa) is your decision however, it provides a far smoother experience for your site visitors.
The technical argument for using WWW is that it represents a web server within a domain. Like many such things in networking, as in other fields, this is a convention and not a requirement. While there may be no other servers within that domain, it provides a rather explicit indication of the server purpose (even if not running on a dedicated system). It also provides an explicit address for a domain's main site location, especially when a domain exists with seperate sites on other subdomains and offers a logical consistent partitioning of domain namespace.Edited to correct typo
Edited by NyteOwl, 10 September 2007 - 02:20 PM.