Hope no one minds if I chime in but a benefit may be that it gives you some intelligence points as you should be benefiting from all the ladies combined intelligence when you finally join them.
Also you maybe can have all the other groups turn hostile to you after that ceremony instead of earlier in the game as it is now.
Plus maybe you can get some kind of GP banner that you carry after that, don't know if that can be changed though from your original one you start with.
Maybe you can have some kind of special Gaurdian party Polearm or armor (polearm would be better as thats their signature weapon) that you get when joining them as well that is only available at that time and nowhere else in the game.
Lots of ideas in this head but who am I to put them forth as I can't even dream of making my own mod let alone tell anyone else how to do theirs.
Don't even know if some of the above mentioned can even be done.