In my view there are currently 3 primary reasons why I want strong multicountry vassals.
1. Rasing armies and fleets. The new system really cuts back on micromanagement to the point that I practically never raise my personal levies anymore (please allow us to raise all our personal levies in the same way we can raise our vassals levies). I always divide the Kingdom of Jerusalem evenly between all my major vassals
2. Tech efficiency. Without the ability to exploit the demense tech bonus your realm, outside your personal demense, is simply never going to develop technologically.
3. Economic efficiency. Vassals do improve their holdings just fine, provided they have a sufficient income to build stuff. Increasing the average income of your vassals will allow them to get started sooner. The only real weakness with the build priorities of the ai is that it seems to prioritise the cheapest always, so universities are rarely build.
1. Rasing armies and fleets. The new system really cuts back on micromanagement to the point that I practically never raise my personal levies anymore (please allow us to raise all our personal levies in the same way we can raise our vassals levies). I always divide the Kingdom of Jerusalem evenly between all my major vassals
2. Tech efficiency. Without the ability to exploit the demense tech bonus your realm, outside your personal demense, is simply never going to develop technologically.
3. Economic efficiency. Vassals do improve their holdings just fine, provided they have a sufficient income to build stuff. Increasing the average income of your vassals will allow them to get started sooner. The only real weakness with the build priorities of the ai is that it seems to prioritise the cheapest always, so universities are rarely build.