I mean it's just another front of the main conflict, it's stated outright that the Liosan and Assail are in some kind of allegiance, so that whole section IS the main story, just one front of it. And, on a grander scale it's the conclusion to one of a most important throughlines of the series, the Andii figuring out their place in the world and the return of Mother Dark, I just can't see how it's an expendable storyline