It's quite useful to be able to restrict the listing to the main namespace, the project namespace, or the template namespace.
Namespace filtering will not work for custom namespaces, since their IDs are completely random across projects. But even for those it would be useful to have to option to list only, or exclude, talk pages.
In the light of upcoming T487: RfC: Associated namespaces, don't rely on talk namespaces having odd IDs. Rather use MWNamespaces::isTalk / MWNamespaces::isSubject / MWNamespaces::getAssociated