The secondary role system is over-rated. The three main roles for each character have abilities that characters without that main role can't learn.
When I was power-leveling, I unlocked Army of One in Lightning's Ravager role. I glanced through the roles of the other characters and I noticed Hope and Snow each have a move that no one else can learn. Does Vanille, Fang, and Sazh each have a special move, or are Lightning, Hope, and Snow the only ones that have special moves.
Also, why do only Lightning, Snow, and Fang learn Launch? Sazh has Commando as a main role along with the other three, but he can't learn Launch. Is it because of their choice of weapons?