Wikinews:Why "Portal:" is good

Fundamentally, namespaces have meaning. If we do violate that meaning, we're going to run into problems in other areas where it is interpreted. Categories have the Category: namespace, Wikinews has the Wikinews: namespace, Images have the Image: namespace, and articles have the Main namespace (no preceding text). It just makes sense for Portals to have a respective portal namespace. This namespace has been created, so now it's just a matter of moving the portals over to it. It's a good idea from an architectural point of view to maintain semantics of a namespace.
Another reason for putting portals in the Portal: namespace is to have a separate article count. We often publish the overall number of articles. But this includes the ever growing number of portals and is therefore misleading. By having their own namespace, we can publish statistics for portal pages only as well.
It simplifies the process of creating portals, and automatically filtering them from relevant lists, as adding a category is not necessary. Just putting the page in the right namespace is sufficient.
It also affects the Special:Allpages display. If they can filter by namespace, readers no longer have to view these pages when looking at articles.