The glass screen may be scratch resistant but it's not scratch proof. It will get scratches over time. They may not be real notiable but they are there. I use a screen protector for resale value.
I have had my 3GS since launch date last year and have never once put any form of a case or screen protector on it. They all just look bad. AND, the device is perfectly fine with no scratches on the screen or smudges or anything. it looks pristine.