As you can see, you can use the touch screen in Pikmin 3 as an aiming input and the TV screen will display a reticule. That reticule will move based on how you move your stylus across the screen.

Imagine if Wii U's Rainbow Curse had this feature as another control method! As long as you keep your stylus on the touch screen, you will have a reticule on the TV screen. To draw a line, hold down the trigger buttons (L & ZL or R & ZR). To tap Kirby, put the cursor on him and press the trigger buttons.

This way you can won't have to look at the GamePad all the time to draw precise line!  A reticule will guide you!

Or we could just draw with the Wii Remote >_<

Edit: If you're unsure about Pikmin 3's controls, games like this exist as FPS/TPS on the DS handhelds like Kid Icarus

No! Just no! Drawing on the gamepad is better. I guarantee.

But you agree that the game should let you draw lines accurately without looking at the GamePad screen, right?

Yeah :/ but how can it be as accurate as the gamepad?

It's pretty accurate in 3/DS games like Metroid Prime Hunters and Kid Icarus Uprising. I just chose Pikmin 3 because something like this exist on the Wii U already.

And that control method in Pikmin 3 wasn't too bad either. It lets you transition to the map screen with less hassle too.