Problem is, as a realm conquered more ducal centers, controlling more lands would be easier.
Large realms in areas of mostly rurals would be identical as it is now. Large realms in duchy-dense areas (FEI, southern EC, parts of Dwilight) would be much easier. FOr example: under this system, D'Hara's maintenance issues would be greatly simplified, while Morek would probably enjoy little benefit.
The thought was to add the duchy loyalty to non-duchy seats, though. As most of D'Hara is ducal seats, it would only change the maintenance for the townslands and rurals.
Though that's something I hadn't considered.