The case is the business case. Wii didn't get AAA 3rd party support because it was a low-def system with weak online that couldn't handle PS360 games, and the combined PS360 userbase was enough to justify bringing them their. In addition, on a Nintendo platform you need to compete with Nintendo software, which makes it a tougher sell.
On the Wii U most games could be easily scaled back from PS360 to play on it. What Wii U needs is a large enough install base, and these games need to sell in enough quantities. The first batch of AAA games mostly broke even with 100k+ sales (AC3, COD, B:AC). But that's still low, but expected since most of the ports were late and on last gen hardware. If the next batch (AC4, WD, Destiny?) does well (500k+ sales) then you can bet that third party support will continue. Most games will be built to run on a range of PC's and consoles to hit as many consumers as possible. Targeting only high end doesn't make a good business case.
What Nintendo needs is just to sell a lot of systems. If there are 10 million WiiU's out there and only 3 million of other systems, even if the game sells at a much lower rate it could have a significant portion of sales. If WiiU gets a price drop to $299 and other systems are $399 or more, Wii U could make it into many more homes.
This Christmas will be a big test since Wii U is getting AAA games at the same time or before the other systems and it will have a larger install base. For Wii U to have future support, these games need to sell well. Looking at Wii for reference doesn't apply because it was so technically far behind, and it simply didn't get ANY AAA 3rd party multiplats, while Wii U has gotten and is getting them.
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