I thought the second screen was stupid when I was playing demos in the stores too. Now that I actually have one and have played through a few full retail games, I see its usefulness. I actually have started to really like it. Especially scratching those ticket things in Rayman..
In regards to the DK game in Nintendo Land. The gamepad simply offers a closer view to your character than the TV screen does in that mini game. There's no reason to look back and forth. Choose which zoom you like better and stick with it. My girlfriend preferred looking at the TV. I prefer the controller.