Or is it there just for wii backwards compatibility? I was thinking (again) games like Xenoblade X and Zelda U are going to be pushing the limits of 25GB of space. Since the price of SD media is fairly cheap could a company pack in a pre formatted card with additional content that has to be inserted to play. I am aware that day one patches also can remedy this issue but that seems less intuitive than simple add a card in the box. Like adding a 8GB card in the box.
But I don't think the SD slot does anything for the U anyway so it may be a moot theory.