Monopoly on recruitment and highly defensible position are the only benefits I see now.
But it costs a lot more troops and gold to take a castle, and unless you are able to then reinforce the castle with more troops and better equipment than the AI, then you just basically gave away the castle to anyone smart enough to attack.
Having a defensible position and being able to recruit more troops is nice, but it seems a lot of clans have decided it's easier to take the village and try to hold it than a castle, even though the castle would be obviously easier to defend. All comes down to the raw numbers, troops and cost to equip them.
I don't make decisions or have any say in Hospitallers, but I always wondered why they didn't take some more castles over to use as a "last stand" type of strategy (if needed). And let the villages be taken over, and retake on your own terms.