I don't posses some large knowledge about western regions of Gondor, but I do suppose that they are not that much in tune with the lore, but as Rora Lion said, they give a certain feel to western part of kingdom and are, as I said already really useful, and I don't think that they change balance drastically. Haradrim have their own horse archers and now try to imagine slow moving gondor infantry army trying to fight with harad that have combo of spearmen+heavy cavalry really early in the game, and on the other hand, first region that will be attacked by Isengard from the north is Pinath Gellin and Raiders give you nice edge, that you need, when you fight Isen Wargs and pike formations because these things are sure cavalry killers and one thing that you can not afford when you play as Gondor are gross losses in manpower that are more or less guarantied in war that is fought on 3 fronts, and here Riders come in