Not having read the books (well, I'm currently struggling with book 1 but not doing terribly well), I would think you must be right. I'd assumed Robb was going to be a main character. He certainly seems that way in the show.

ETA: should add, it's one of those serendipity things really, like Spike in Buffy the Vampire Slayer. He was meant to be a disposable half-season villain, but the actor was so good they kept him and he ended up being quite an important character. Richard Madden has obviously brought something to Robb that isn't in the books. I wonder where they'll go with it?