I find very promising to be able to map metadata with namespaces. I'm wondering why this limited to tags, rating and comments. Why not the other metadata like City, Sublocation, Province/State, Country and other Latitude, Longitude, Contact, Subject, etc... ?
Trying to migrate from MediaPro, I strongly support this suggestion. As I wrote in a comment to bug 367700, I have a lot of valuable information in image metadata that is otherwise lost (at least for image management), because it is not even possible to search for it in digiKam.
*** Bug 465911 has been marked as a duplicate of this bug. ***