She would have value as a non-opponent. That is, if Souma kicks her out, she could potentially cause trouble. However, Amidonia already peacefully transferred under Elfrieden rule, so what could Roroa offer anymore in the future? Amidonia is already under control, and the people there feel like Souma will give them a much better life. He doesn't anymore need any help to win those people over. He just needs to treat them fairly, solve the food issue, and not tax them to death. Those are things he has handled in the past successfully as it is. I'd say Roroa has to bring something concrete onto the table, not simply her former position as an Amidonian princess. Souma has no use whatsoever for her (former) status or titles. He even keeps insisting he's, himself, nothing but an acting king. He's sticking to his non-kingly, regular clothes as well to underline it, except for rare, very formal occasions.