I'm working my way through XIII, and I have just reached Gran Pulse (so, like, no story spoilers would be choice), but I have one question.

I borrowed my friend's strategy guide (Piggyback) because I was stuck on

Spoiler:
Barthandelus


but I've read a little more of the book. It makes a strong case for only using three characters (Lightning, Hope, Fang) for the remainder of the story, and specialising the other three characters in their best roles (Vanille - Medic/Saboteur; Sazh - Synergist; Snow - Sentinel).

The logic is sound. With Light/Hope/Fang, you have access to all six roles. You can focus equipment upgrades instead of having to spread them. You can keep a stable paradigm deck. With Vanille/Sazh/Snow, you can reach the upper heights of their final Role Levels, where (apparently) the stat boosts become really large. This, we are told, is useful for later Mark missions.

My question is whether or not this is a good decision. Have you tried it? What are the benefits/drawbacks? What conclusions have you come to regrading its suitability as a long-term (post-game) strategy? What about the short-term (pre-post-game) strategy?