I suspect that any class-specific special gear that can be acquired through participation with the Company quests will likely be limited to the guilds that are present in that city. That would be the inducement to participate at each city.

That said, participation with each city-state's Company would be necessary to improve various classes through this gear, so after that it would just be limited by time, and would provide (nicely, imho) a way for classes to specialize from each other just a bit.