I've never owned a Wii U, but I have played a lot of games on DS and 3DS, and I can't think of a single game where the second screen meaningfully improved the experience (whereas I think the touchscreen was great).
The problem is that even on the DS and 3DS, where the screens are relatively close together, you still can only look at one screen at a time (due to humans' relatively narrow foveal field of view) so when you're looking at one screen, it makes absolutely no difference what's on the other screen. In that case, though, you might as well just one screen, and switch between the two views with a button (i.e., the way any game with a map works on any other system). As long as switching views is sufficiently responsive (this is the real trick, since it's often not true) then the second screen conveys no advantage.