Plus, sometimes you want an Internal: namespace that cannot edited by just everyone. (Maybe because it sets the guidelines for editing the rest of the pages :)
Having one namespace protected is very different to having the entire site protected.
Speaking of that, what is the easiest way to inhibit editing of an entire namespace? The blacklist extension seems to be able to handle it, but I guess it will just build a giant whitelist of all other pages to set them to be editable?
Use $wgProtectedNamespaces in LocalSettings.php (examples in DefaultSettings.php and on mediawiki.org).