I believe this is Robert Jordan's foreshadowing of Siuan's future. In that particular scene, she channels a sword made out of Air upon Nynaeve's remark that she's rather learn to use one. Siuan, while astounding the two girls by making the sword appear out of nowhere, further says that a sword is almost useless. She then turns it into a paring knife, saying that the knife was more useful. In the future, Siuan Sanche realizes she's more useful with her reduced status than before.