User blog comment:B214/Volume 23 discussion blog/@comment-5480911-20170213225853/@comment-80.180.81.89-20170216010115

And Rassei will be a very strong dragon, even Ophis already said she wants to train him, and maybe she will get other dragons (maybe the rare spectre dragon that now is still in his egg). If Asia finds a way to comine her dragons power with her sacred gear (a SS balance breaker) she could use her dragons power without summoning them, so she could use them in rating game without problem.

About Rias choosing longinus users... Lint and Valerie are not in her team permanently and I doubt that, even if Rias improved, she could reincarnate two longinus possessors and talented with a normal Knight and Bishop piece. And about Gasper, she didn't know about the potential of his sacred gear and about Balor's consciusness inside of it, no one knew it, neither Rias. She was lucky here, exactly like with Issei.

By the way, even if Rossweisse was offered by Rias, Issei chose to take her. And she is an incredible power up, her defensive and offensive abilities are great and her ability to create powerfull spell (like the one which should have sealed the Trihexa) are an incredible weapon for everyone who has her on his own side.