I believe the primary reason they showed off demos using Wii Sports Resort was timing: it's fairly evident that the games in development for the 3DS were not that far along, and even in order to get a couple minimal demos together, they had to rely on existing assets like Wuhu Island (or from resurrected N64 games like Star Fox).
That being said, Wuhu is the perfect setting for Pilotwings, and the inclusion of "Resort" in the title indicates that it's going to be central to the new game, not just one map amongst others. As for Mario Kart, on the other hand, I'd say that it's possible the Wuhu track won't even remain in the final game. That's not to say that it would be unusual if it did stick around; they already integrated Mii's in Mario Kart Wii, so Wuhu is not all that out of place.
As for the topic of making Mario Kart more like Smash Bros by having more general crossover material: yuck, I'd prefer it avoid that at all costs, personally. Mii's and locations like Wuhu are one thing, but I don't want the Mario universe to suddenly start intersecting with Zelda, Metroid, etc. It would ruin the identity of the Kart series for me.
I'm looking forward to racing around it
