I don't understand why "party games" are always used dismissively when talking about making use of the system's features. Nintendo Land is an amazing showcase for the GamePad, so is Game & Wario. You see these games using the most novel concepts because they can be free in expressing their ideas with these titles and don't have to hang a whole 30 hour game onto them. ZombiU has you covered when talking GamePad in a traditionally styled game. The game wouldn't be anywhere close to being the same without it. It's also absolutely essential for Pikmin 3 since, as you mentioned, it is an crucial companion to the Wiimote&Nunchuk for optimal play/management experience.
But just like with the Wiimote, it's not always about completely building games around this one thing, it's just as important how it can be used to make traditional stuff better or add new layers on top of it. That's where NSMBU's boost blocks come into play. It created a whole other gameplay style in a genre and franchise that is 30 years old!
A select few of the third party developers have also managed to show off how the gamepad can make existing games even better with very simple additions that you just can't do on regular controllers (Deus Ex HR Director's Cut [drawing notes on the map, recording voice messages and sharing them online via the mic and tagging them to the map, gyro aiming], Mass Effect 3 multiplayer [push-to-talk feature], the CoD and LEGO games [two screen multiplayer], Assassin's Creed & Monster Hunter [HUD-free gaming without losing the information], NFS Most Wanted U [selecting cars quickly, world modification, creating local co-op where there wasn't one before].
The GamePad has been justified and shown off countless times already.
You see the glowing red thing to the right? Communist! Get 'um!
