Chozo Ghost, they could. But Natal is supposed to eliminate the controller...to make it feel more natural, not like you are playing a game where you must pickup, put down controls, that misses the point.
The Wii is different. Nintendo, never attempted to eliminate the control or even the notion of using a control...but modified the concept of a controller to use motion as an input device. In fact it seems Wii actually has made the controller more apparent in its games and philosophy. You must know what games use what controller and how you want to play it.