This is the way I interpreted it reading book 1: every POV was chosen to provide a contrasting view of the world and it's events. Jon obviously needs to be the eyes in the North. Bran, Arya, and Sansa view the world through childhood innocence, with Bran staying up North having to deal as a cripple and prepare for his own different kind of story (foreshadowed by the 3-eyed crow), then Arya and Sansa providing contrasting viewpoints in Kings Landing. But Robb? He's really his fathers son from his perspective the chapters probably wouldn't be a whole lot different from Ned. So instead Martin uses Catelyn as the POV, giving a window into the world from a grieving wife and concerned mother. So rather than reading about Robb swinging his sword and kicking some ass, we read into a woman's role in this world where Catelyn can do nothing but sit safely off to the side out of danger and pray for the safety of her son off at war. In my opinion HBO captured this quite brilliant in episode 9, almost brings a tear to the eye. She's still in a position to advice Robb though and let you know everything that's going on with his war efforts.