I see the pros of Gavelkind succession and Primogeniture succession as well as their cons, Elective sounds good in theory and I've used it to elect the better heir, but I honestly can't see any pros for the Seniority laws because to me it Primogeniture except for the part that your brother would inherit (as an example) and not your son. When do you want Seniority and when don't you want it?