The reasons why I want the Game Cube adapter to work in Wii mode on the Wii U: Active Life and Metal Slug Anthology. The Active life controller is the best combined workout and fun factor if used right. (The Balance Board can sometimes be fun, but is not as aerobic). And Metal slug plays awkwardly on a Wii U becuase a) you can't use the classic controllers, and without the Game Cube Ports working, you can't use Game Cube controllers on it either. You'll be motioning for buttons, and pressing buttons in weird places. Then and only then Nintendo can claim 100% Wii compatibility with Wii U.
With this you can make exergamers that are actually games, not just computer gym trainers. Games where my actions affect yours and your actions affect mine. Make a Run And Gun Augment for Splatoon, using the Active Life Pad to determine the speed you run at, and the joypad for direction. Makes for a more realistic shooter in terms of not being able to go all out all the time. You have to pace yourself, and pick your moments when you're going to dash.
The Balance board is fine for a Mario Pedal Kart Augment. I Biked on the Balance Board and it was good exercise, but there may need to pedal speed adjustments so the game has the same speed-feel as Mario Kart 8.