The question is, who will buy such multiplatform versions for Wii U when people can play them on a console they already own?
I never bought this line of thought. It seems to be asking the wrong question entirely. Of course PS360 multiplats are not going to be the main draw of the Wii U. But nintendo showed everyone this generation that there is a massive market for their products, one that is considerably larger than the individual market for either rival platform. And though nintendo has waned considerably these last few years in casual appeal, the mainstream mass market has continued to grow considerably as other competing products began to target those consumers directly. That market still exists and it's larger than ever, and nothing is stopping nintendo from releasing a product that once again grabs their attention.
The primary reason to purchase a Wii U is going to be software unique to that system. This should be painfully obvious, and phrasing the question strictly in the context of multiplat software is nothing but a thinly veiled attempt to downplay the console's appeal. However,
nothing is ultimately going to stop Wii U owners from buying those cross platform games on their new console. People aren't going to buy Wii Us for CoD. That's obvious. But why wouldn't Wii U owners buy CoD for their brand new Wii U? Nintendo is going to end up selling 100 million Wiis. And while they may or may not ever reach that same figure with the upcoming successor, what should be clear is that there is an enormous market out there for nintendo consoles. Even if a lot of hardcore gamers were left disgruntled over the lack of certain types of content on the Wii, there were tens of millions of consumers who had a great time with Wii Sports and Mario Kart and plenty of other wii specific titles. Nintendo is trying to market to
both demographics this time around. Some games will definitely have longer staying power on PS360 because of XBL/PSN communities, but that is not going to be the end all be all determinant of which platform consumers decide to buy the latest release for. In such a scenario,
we're already assuming people have bought a Wii U for console specific software. People are going to want to buy software that justifies the purchase of their brand new console. Add to the fact that the Upad offers an easy way to distinguish between Wii U and PS360 versions of games and you should realize the question should be "Why
wouldn't WiiU/PS360 owners buy multiplat games for their new and unique system?"