Too much math to be honest. With the current system we have potencies (almost) balanced per level, while including level synced stats. With your suggested changes, it would require ridiculous volumes of math per class, per available skills (and levels because the coincide), in addition to coding figuring out whether or not people did their class quests and the types of debuffs/buffs each class provides...

Like balancing a level 15 Gladiator's eight skills (including role abilities) against lets say... A White Mage's 26 skills or... Red Mage's 28 skills (excluding consolidated skills and role abilities) is very tedious and also very annoying with the amount of different variables, party comps and alike.