I think you're right. There is not codeing for the new forms of kyurem in B&W so why would they make a perminate form of a pokemon already released? If you can't trade pokemon within the same gen. Why do it?
Is the 5th gen your first gen? Platinum had Sky Forme for Shaymin, and Giratina had the Origin Forme. Platinum is the 3rd game of the 4th gen, but they weren't tradeable to D/P. Now that Black2/White2 are doing it, why are you saying "Why do it?" when Gamefreak has already done it just a few years ago??