There's also cross-buy in Europe though. No idea why it's not been implemented in NA yet.
No clue either. But maybe this is a way to make that sort of thing easier to do. I'm really betting on a unified service from what Iwata said back in January of last year about moving to NNIDs and moving away from a device based service to an account based one, and what better way to showcase this than Mario. It would make the feature more well known and that it can now be done globally.