Last modified: 2011-04-14 15:10:19 UTC
What would be useful, in addition to being able to select individual namespaces from the drop-down list on pages like [[Special:Newpages]], is the ability to select all content namespaces at once. Perhaps this could be extended to a meta-(talk) selection, too, but I haven't given much thought to that one.
See bug 12689 to see where this idea was brought up.
In some instances this isn't really feasible without schema changes, where the indexing is by namespace.