I'm all for Horizontal growth along with verticle growth, it's just hard to find a proper balance so the horizontal growth "If added before a final"Ish" level cap is placed from becoming overpowered is rather hard.

Merits were easy to balance because for the longest time the level cap was 75 so it was a way to customize your character beyond his "Natural" limits. If they decide to add this kind of growth they need to do it carefully and possibly make the growth be something beyond just SP.

I suggested something like it earlier as an "Accomplishment" system where by accomplishing certain feats you became more proficient with certain items/skill/~