If you are in normal status, if your 485 is refused, you are still in your normal visa status, but if you use EAD, if 485 is refused, you must leace USA, and can not automatically return to your previous visa status.
Now your 485 is pending, althouth your 485 is not be refused, but you still can not return to your previous visa status within USA. This is the shortage of using EAD.