I should rephrase: The real issue here is that not all "barony" holders get married/have children. If you're playing the christians, it doesn't matter, because the bishoprics have their own dynamics and don't get taken over by their feudal overlords. However, as a muslim feudal ruler, you can hold "bishoprics" (or whatever they'ye called), hence a significantly raised likelihood of vassals quickly increasing their demesne, if their "bishops" die without male heirs or do not marry.