User blog comment:B214/Volume 22 spoilers/@comment-182.53.88.122-20160724082440/@comment-9595671-20160724084233

Yeah, once the tournament ends if Rias were to ever participate in a game, Issei would still have to join in as her pawn. Now he can simply enter games as the king with his own peerage.

Plus I think the reason he can use his DxD form in a rating game is because unlike his CCP, it's not connected to his Triana.