Early on, levelling up a role gives you new abilities. And all level ups, regardless of role, give you attribute increases (which are somewhat related to the role you chose). But once I stop getting new abilities, what is the difference between being a level 60 Ravager and a level 70 Ravager? Does my level affect the effectiveness of skills, separate from my overall attack/magic attributes? Does it affect the bonus that other party members get based on my chosen role?
Once you get all the abilities for a role and the relevant expansion bonuses (bonus boosts, in particular), there is no effectiveness increase for role level gains outside of stat increases.
While you get small increases to stats no matter what node you choose, the main benefit to leveling up a role once you get all of its abilities is the bonus to your attributes you get when you level up on a large node:
If you wanted to min-max, you'd only level up your primary roles that correspond to the stats you want to maximize on large nodes, and leave the small nodes for the roles you don't care about. This should wind up providing a mostly-even leveling experience across all roles, mitigating the "nothing left to get" problem you have by focusing on only one role at a time, at least until the late end-game.