Quite the causality problem. Is their concept so bad they've failed to utilize their controller in great lengths in many of their games or is their execution so poor that they've failed to utilize their controller? I'm going to go with concept, Translating iOS/Android touch based games/concepts to consoles is far more difficult than I think even Nintendo anticipated. The stylist translated well enough though =/.
Don't get why people see touch pad and automatically think Nintendo was aiming toward replicating iOS/Android gaming? Nintendo doesn't and hasn't even tried doing that with the Wii U GamePad. They have tried it with more then a few of their eShop titles on the 3DS but the Wii U, Nintendo's main focus in regards to the GamePad (I would argue to their detriment) has been accessing the Miiverse and playing the games on it.
They have WAY too many games that play without it for anyone to say that touch based games/concepts were Nintendo's primary goal with the device.
Then why make the pad? For asynchronous gameplay that hasn't panned out? Their major titles are coming out later this year and early next, none of them are innovavting that touch pad. That's going to be going on a year and a half for the console's life, plus whatever time the console was in development. BUT as I was typing that I remembered Nintendo's console strategy; which is their hand held strategy. The tablet controller is the console version of the DS's second screen. That second screen hasn't really been too useful for gaming. Which goes back to poor concept.
I agree they have failed to give compelling reasons for why they have the GamePad on the system. The best thing they have had has been the ability to play games on the GamePad but that in of itself isn't enough as most would attest.
But I disagree in it being the concept that is the problem. Going off the success of the DS and 3DS, having a second screen with touch capabilities has been shown to work, especially with certain games. The issue with the Wii U has been Nintendo hasn't released any of those types of games on the system, along with really pushed it as they and other developers did the second touch screen when the DS first came out or motion controls with the Wii. They haven't released anything like Brain Training, Nintendogs or Wii Sports to make people take notice and they failed in implementing a working multiplayer that doesn't give two or more parties equal footing because only one GamePad works on one Wii U for most multiplayer games.
The concept is fine, two screens haven't hurt the DS/3DS development in anyway once developers figured ways to implement it into their gameplay or work around it, even Microsoft (LookinGlass) and Sony (VITA intergration) are dabbling in it. The problem is execution and as you put out is that Nintendo has failed to put out software that uses the GamePad in innovative or even unique ways. Even the other new features of the Wii U (Miiverse and Nintendo TVii) that Nintendo was hyping before its release are things that can or will be accessible on their competitors systems and they didn't have them ready at launch.
The concept is fine and for the most part proven successful with other systems to different degrees, Nintendo just botched it in nearly every way when it comes to implementation and presentation to the public.