the +1 advisors is actually useful you want to hire a specific MP adviser to do things its sort of like picking your own Idea of +2% missionary strength or if you need dip reputation for dip annexing or the army morale one
It is useful. But I think it's one of the things that anti-synergizes with itself: the more advisors you are able to choose among, the less useful it is to have one more.
(The other thing I think is anti-synergizing is heir chance: stacking those +25% heir chance ideas should also be hilariously weak, since they are fixing a problem that's getting more and more irrelevant)