The two main reasons I can think of are:

1. You can't sing or play a note yourself.
2. You wrote it in the opposite gender.

A reason not to hire has already been mentioned (they may not sing it not as you want it). Pick your demo singer carefully.

Vic

It's never too late? Yes it is, so do it now.

If, given time, a monkey can write the complete works of Shakespeare maybe there's hope for me.

http://store.cdbaby.com/cd/vicarnold2

http://www.soundclick.com/vicarnold

http://soundcloud.com/vic-arnold