Some Wii U owners probably won't be too happy though. The Wii U is sorely lacking in third party exclusive content, and this could have been one of those games that would have pushed a person to buying it to replace their main console being their 360 or PS3. Instead, this title is going multi-platform, essentially pulling a Rayman Legends (that's the one, right?), and not feeling secure enough in only being a Wii U game in terms of potential sales. Moves like this, although nice for everybody else in terms of available content, and even if it wouldn't necessarily have been a system seller, shine a poor light on the Wii U. It tells people that developers don't believe that the Wii U can generate enough sales on its own to make the game worth releasing only on it. That tells other developers that hey, maybe they should consider last gen or next gen hardware to develop on that isn't the Wii U rather than focusing on Wii U exclusives.
The third party presence on the Wii U has never been strong, and it has only been going downhill as developers have been getting scared by the system sales not jumping. Nintendo is having a hard time getting consumers to properly differentiate between the Wii and Wii U still, and this is causing a further lack of proper third party support which could cause people who do know what the Wii U is to not purchase anyways because no third party games.
In the end, the Wii U needs more than first party content. I know that's how Nintendo has sold systems in the past, but anymore, third party developers are a very important point to consider when looking at potentially purchasing a console.
Now, the Wii U may bounce back. I feel like it will. It isn't a bad system. Nintendo is just suffering from goof up after goof up when trying to sell it. This is going to cause more situations like this to crop up in the future. If you look at a list of Wii U games, both upcoming and released, the exclusive list is already very small, and it shrinks to maybe 10 games when looking at third party exclusive content. It shrinks even further when considering decent exclusive third party content (maybe two, three, upwards of four games?). The bad part is that this includes both announcements and releases. Nintendo is also pushing out a lot of first party content within this first year in order to try to sell the system, and I fear this is going to kill their safe zone buffer if these titles don't sell enough systems to pull the third parties back in. I mean, their Nintendo Direct for E3 didn't do much to alleviate the fear of third parties not really being interested.
What this boils down to is that the loss of third party exclusives, even ones that may not have necessarily been system sellers really hurts. It sends poor messages to consumers and developers alike, and leaves more room for Nintendo to fill in the gaps in the mean time.