Let's be honest, you don't want customization you want optimization. Customization implies opportunity cost, meaning that if you pick one perk there's an equally good perk that you are now disqualified from. The scenario that I've just stated never happens in a customization system because everyone just picks the perks that best benefit their job because it's too difficult to balance every choice to be equal especially in a game like FFXIV where there are very few stat choices that you have to make.