Here's the thing about that. Wii U now functionally has BluRay type discs, with a single-layer 25gb storage capacity, and given the possibility of dual-layering, maybe someday even 50gb. That's a hell of a lot of memory. The reason that PS3 games especially, often (but not always) have you download part of the game data to the system, is because reading all that data would take a lot longer. It's basically to try and eliminate load-times. Especially when you consider that 360 and PS3 disc drives are older. Wii U's may not be THAT much more advanced, but I'm willing to bet it's a faster disc drive, and despite all this talk about "slower" ram, the system itself is, by and large, STILL more powerful than the 360 or PS3.
Regardless, there is a good chance that some games will have you download part of the data, to help ease loading times, etc. Maybe not, but with discs that big, it's something you always have to keep in mind.