you obviously dont know much, the purpose of the Demo is to show how the song is suppose to be sang, the key your suppose to be in. go listen to Ne-Yo's demo of cant be good for Janet, or Floetry Butterfly for MJ and you'll see they sang it exactly as the demo.

the demo is how producers and writers present there songs to artist. go and listen to a demo Chris brown presented to the pussycat dolls and you will notice he sang it as they should as a female to a male.