Same as always, controlling the world. Rome will just be an economic powerhouse province and your control of it will probably lessen the chances of rebellion. The main reason for controlling Rome will be already developed provinces. Controlling the Republic will and should be an asset, not a game winning move. As for the Praetorians, they probably will fight well and beat normal legionairies but the hint "not suited to the rigours of campaigning" probably means that they wear down and get tired easily, making them a temporary power-house.