I have to disagree to an extent, FF13's crystarium and party systems are so much superior to FF13-2. I mean its linear in FF13-2 and now you are forced to tame/capture monsters for party members even though theres not many choices when you get to endgame because certain monsters are just the go-to type for a paradigm. I do love the cutscenes in FF13-2 and i really don't mind the QTEs, but it gets alot better with the DLC episodes. Those are challenging, and you get nice party members.
Each Role in the Crystarium you can level up to 99. Certain abilities are learned at specific levels. The relevance of the actual sphere board is that the large spheres are what grant the large stat bonuses. If you are one of those types who wants to maximize everything, save the Commando and Ravager level-ups for the big spheres; those are the roles that grant large Attack and Magic boosts, so they're the most valuable ones. You'll be getting a ton of experience--the game is very liberal with it--so such a strategy only hurts short-term, with a large payoff.
I'm not sure how clear this explanation is--ask away if you need anything more.