Yeah, for the Wii U it was an awful idea. The whole premise revolved around you having to take your eyes off the screen and look down and it was never used for anything important. Those people who claim it was absolutely critical in Splatoon just make me laugh, you couldn't afford to spend more than half a second looking at that map because it meant taking your eyes off the gameplay, as a transparent overlay you can play and look at the map at the same time.
For the DS things are a bit different, the two screens were right on top of each other and games rarely required you to look at both at the same time. On that system alone, developers made good use of the dual screens, problem is that by the time the 3DS rolled around all those ideas dried up and it went back to being a map screen again.