I am also of the view, as some others, that they're saving Nerva as the "inside" contact for Garlemald, especially as his family had ties to the Populares. Gaius has been put to other uses, and Maxima seems to be more of a bridge character.
I would have to guess that the bridge character won't be Nerva himself, but rather his kid Titus. If Nerva is either dead, tempered, or just uncooperative, Titus would be the last remaining heir to the Garlean throne.