It's non-linear because you have to find out the weapon subsets that give you Synthesized abilities, and you have to figure out what weapons are perfect or situational for each character, example, Vanille's debuff staff is great for the Death trick, but useless on many enemies that have wide debuff resistances. Or with Lightning, you could go straight to Helter-Sketler because it's clearly a strong weapon, but then you'll miss out on the fact you use her as a Ravanger for such large portions of the game, where the Axis Blade would allow her to constantly attack and stagger the enemies incredibly quickly, along with interrupting their attacks while COM's chain their Attack commands together.

Also, I lul at all the people quoting FFXII now. Square gave FFXIII due negative responses to the pure vast openness of FFXII, oh how the tables have turned.