Last modified: 2008-11-04 21:43:28 UTC
__NOINDEX__ is available for those circumstances where a page needs to be de-indexed for scary scary reasons. There is absolutely no reason to do this by default, anywhere. With MediaWiki's crippled search engine it makes pages in de-indexed namespaces literally impossible to search, at all, which is unacceptable.
Other than a small handful (as in, about 5-10, at most) special pages that have this hardcoded to 'noindex,nofollow', all of these are configurable on a per-namespace basis (see http://www.mediawiki.org/wiki/Manual:$wgNamespaceRobotPolicies).