From a fan perspective / Wii U owner: At first I was rather bewildered by it. Especially since the Wii U version isn't coming out 1 month later, not 2, but potentially 3+? Yikes, and the Wii U probably needs it more. It will truly be a bummer that I'll be seeing the roster and gameplay, but won't play it until at least 3+ months later... Luckily, the positive thing I can take from the mess is that Mario Kart 8 will surely keep me busy for quite sometime anyway.
From a business perspective: Look, I really despise what they've done, but it makes sense as well. Mario Kart 8 will be releasing on May 30th. MK8 has been steadily hyped for quite sometime now and will, in itself, pull consoles off the dusty shelves; this is a fact. Next we will be seeing Sm4sh 3DS a few months from then, raking in EASY and fast cash from an admittedly more popular and cheaper system on a massively hyped title (especially once E3 hits). A few months later, possibly in November-ish, we'll hopefully be seeing Sm4sh Wii U come out. It will probably be bundled up with Wii Us, as will Mario Kart 8, and will probably pull more consoles off the shelves. There is a notable spread in the releases here, and this is for the best interest of Nintendo to make money. Releasing both Sm4sh Wii U and Mario Kart 8 so close would have likely hurt in the long run since there may not be a notable release in the winter of 2014. That being said, I know multiple people who will have no problem rushing to the local Wal-Mart in late-May to prepare for Mario Kart 8, so perhaps the notion of having both Sm4sh version close wouldn't cause problems, either.
Essentially, Nintendo is clearly banking on the notion to milk everything they possibly can out of Mario Kart 8, then milk everything out of Sm4sh 3DS, and then bundle up Sm4sh Wii U for the Holidays for optimum sales. In the long run, I do believe that the Wii U version will certainly take more time in general to develop, and when November rolls around and Sm4sh Wii U does come out, I think we're all going to see which is the better game, and will probably be, the one that the competitive scene will choose to flock to; I firmly believe the Wii U version will be of better quality as well. We will be seeing many people buy both versions, and Nintendo knows this; it's all part of business.