I think this gorilla glass they use is supper tough.
Even if it hasn't progressed in 3 years our old iPhone 3G which is now soley used by my 3 year doesn't even have scratches on the screen.
Previously it was my wifes phone and she dropped it god knows how many times.
Scratches on the back but not the screen.
I'm too scared to try without. Just the other day, my iPhone 4s fell off my leg and down between the seat and the central console in my car. I practically mangled my fingers trying to retrieve it and somehow the screen was scratched to hell, i think as i struggled to pull it up the screen rubbed against the metal railing the seat fixes onto.
Luckily as i peeled off the screen protector the screen was pristine. However, who knows what would have happened without a protector.
I personally don't even know it's there, it's well installed, no bubbles and perfect alignment with the earpiece and home button.