Drop a dollar and you can use the 3DS as a controller without owning Smash 3DS.
Hidden in Sunday's Smash Brothers update was an option to use a 3DS as a Wii U controller without the 3DS game.
The application, available on the 3DS eShop for $0.99, replicates the functionality of the 3DS to Wii U control option. Owners of the 3DS version do not need to download this application.