Ever since I managed to wean myself off "map-painting" in the Paradox games, my endgame has been to carefully rearrange the political map of the world, acting less as a world conqueror and more as someone rebalancing the status quo (Napoleon-style) and watching the nations I created rise and fall. Also, there's sometimes a case for creating buffer states or not defending distant provinces that, for example, have been taken as part of a crusade.
I realize that the concept of buffer states is a very non-medieval thing, but looking at crusader kingdoms, it would at least make sense to grant those independence. I can see a scenario where I, as a ruler, would want to simply convert the population to my religion and leave the rest to a local noble. It would be excellent if we would be allowed to do this here as well, especially since every province is already technically capable of becoming its own realm.