On Wikipedia, there is the following problem: Often, if a new article is badly written or unfinished, it is moved from the article namespace to the user namespace (as a subpage of the creator's user page). This is a compromise which ensures the quality of Wikipedia (as the bad article is no longer part of the encyclopedia) and gives the user the chance to improve the article and then move it back in ns-0.
However, pages that are moved from the encyclopedic namespace to a user namespace still appear in categories, that are reserved for encyclopedic articles. This shouldn't be the case, as those articles where moved into user space to remove them from the encyclopedic sector.
Therefor it'd be useful to have a magic word, perhaps something like: NAMESPACEZEROONLY, which restricts certain categories (such as Category:Articles) and their subcategories to pages in ns-0. User pages would still have the link to those categories, but they wouldn't appear in them as long as they are in the user namespace.