A backgroundshader is used when you are making use off images in maya(imageplanes), just regular jpeg and such.
Now when you model something that you want to be in the picture, it normally would'nt cast shadows, but when you create a plane and align it so that it matches up with the picture, and drop a use backgroundshader on it , there will be shadows.
So i would use a regular lambert if you only want to see the shadows that is.

Hope this made sense, good luck