that's not the point. if ubisoft would sell rayman for wii u now and for ps3/360 in september, the 360/ps3 versions would sell less as they will when wii u version will also release in september because otherwise the wiiu version will be already much cheaper and 360/ps3 owner will say "i don't pay full price for a port when it's already that cheap for wii u"
that's exactly what wii u owners said about the ports like batman or mass effect on wii u, they won't buy them if they only cost much less for 360/ps3.
so, even if they will sell maybe 30% less on wii u in september, 360/ps3 version will sell better because of that and overall it will sell more as if it would have two release dates with half an year between those dates.
it's not as if a company like ubisoft doesn't know better how to sell their games as some disappointed fans know. i understand that wii u owners are "angry" but if they want to see rayman succeed (which every rayman fan should hope) they should understand ubisoft and the logic behind that.