Ever since I got into web content management, I have advised my clients to avoid the word "user." It's a useless word because you are never quite sure if someone is talking about a user of the CMS, or a user of the website. For this reason, I get my clients to adopt the words "contributor" and "visitor." A contributor is a person that contributes content or participates in the content workflow of the content management system. A visitor consumes content on the website.
The primary goal of a content management system is to mediate between these two populations. If a CMS was only to think of the contributor, the content would be poorly structured, cluttered, chaotically structured, and hard to find. By the way, pretty much every organization has one of those systems — it's called a shared drive. If a CMS only represented the visitors (or other consumers of content such as other systems), it would insist on extremely fine-grain structure for maximum reuse, pristine HTML (sorry, no WYSIWYG editors), and perfect quality (hello 10 step workflows). The CMS applies structure and rules to establish a compromise between these two groups.
Web 2.0-style sites jumble this model up a bit because the line between contributor and visitor are blurring. Visitors can potentially contribute. Web content management systems like Drupal, Plone and many others merge the contributor interface into the externally facing website. These platforms tend to call registered users (contributors and visitors) "members" and, by default, allow members registered themselves. The distinction between a self registered member and a "SuperUser" is just a matter of permissions.
I still think that the distinction of contributor and visitor is useful because members need to wear different hats. Sometimes they are visitors on the site simply trying find something. Other times they are contributors wanting to post something. The CMS is still mediating but instead of mediating between different types of users, it is potentially mediating between the same user in different contexts. It forces the contributor to put in a little extra effort to make life easier when he becomes a visitor. It would be a little like Clippy telling you "Oh, you don't want to use that file name and place that document there because you will NEVER find it again! And while you are at it, maybe you should print that out because you messed with the margins and I bet its going to look like hell." Clippy doesn't do that and we still find him annoying. Now you wonder why nobody loves their content management system?