I think they'll do a special promotion like "register the 3DS version and a Wii U in a certain time frame and get a download code of the Wii U version for free."
And combine that with interesting connectivity features like the ability to play all modes on Wii U if you have both versions.
They want to sell both versions. Different stages, modes and release date.
I only planned to get the Wii U version, but I'm getting both now.
Yeah, casual Smash players with a 3DS will probably only get the 3DS version, but do you seriously think that "real" Smash fans that didn't buy 3DS or Wii U so far will buy a 3DS for Smash and skip the Wii U for it?