Staying in a straight line is a challenge when you don’t have a central point of reference for the controller. This could be a problem when you’re doing a trick that requires careful balance, like sliding across a thin pipe suspended high in the air, so there’s a touch of joystick thrown in for good measure. Exactly how much is up to you - there’s a slider that governs the balance between stick and nunchuk - so you can tailor it to your own preference.
At the default setting, the stick is used to make minor adjustments to your balance, so you don’t have to rely on trying to guess where the neutral point is on the nunchuk. At the most extreme setting, the stick does all the work and the nunchuk’s motion sensor is disabled. Try it after getting used to the new controls and it doesn’t feel right at all.