If Mario Kart for Switch is essentially Mario Kart 9, then maybe it would be a good idea.
If it's an enhanced port of Mario Kart 8, it's a horrible idea.
"Hey guys, buy our new system. We'll even include a Wii U game for your trouble!"
Not happening.
My prediction: if Switch receives a bundled game, it'll be a new IP.
I sincerely disagree. If it is a Mario Kart 9 there is no reason to bundle it, because it will sell regardless. Even (maybe especially) to those who bought MK8 on WiiU. If they won't bundle MK8, maybe they should just forget about it, because i think a lot of people aren't going to buy it again. If you think giving it away for free is a bad idea, then charging money for it has to be twice as dumb. You are also not seeing that it is in Nintendo's best interest to get this game in as many pockets as possible. Both for the software to make money, as well as for the platform itself. Unless, which i doubt, they are releasing a MK9 right after.
They also bundled Zelda Windwaker WiiU, an enhanced port. Nobody made a fuss about it being an enhanced port back then.
But i'm glad you're saying it's not happening. Hopefully Nintendo got your memo. Your argument of "here's a WiiU game for your trouble" is complete BS as well. It's not "just" any WiiU game, it's the best online game, with universal appeal and a big community. You could also stress the part that it is an ENHANCED version of a critically acclaimed must-play game. Enough examples of games that got an enhanced port of an older game, that people wanted to play.
They can do a new IP, but i don't easily see it getting the same universal appeal as MK and a better fit for the hardware. I also don't see something as "Wii Sports" or "Nintendoland" as better value than an enhanced port of MK8 (for your trouble) and i also don't know if it would be cheaper for Nintendo to built a new IP from scratch.