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?
If you have Seniority succession in your realm and in, for example, some duchies, which are being controlled by your dynasty, you will automatically inherit the duchy once the ruler dies, as you are the oldest member of the dynasty. Then you can give the duchy to someone else from your dynasty and you will again get it back once he dies. This ensures absolute control over who is your vassal.
Then again, there is very little chance of getting a long reign. I once reigned for 60 years, gave me +50 relationship modifier. So freaking useful. I can see that weighing up against "absolute control of vassals" already.