Well, there is merit if the duke angles for marriage to MC. Even if he's not the one that will sit on the throne, Arnold is still the twin brother with good relationship with the prospective emperor. He has shown to be capable of being a delegation, he gets the job done after all so contrary to the expectation of being a dull prince, he's a pretty good catch. Also he did not ask for her when by all rights he could have forced them to. Those are very good first impressions.