It depends on how you treat the device and what else you carry in pockets, etc. Gorilla Glass is impact resistant and scratch resistant, but not scratch-proof. About the only way that would happen would be if the screen was made of diamond. Next best thing would be sapphire, like what is used in high end watch glass, as that can only be scratched by diamond, but it is brittle and can break from impact.
So the best advice is if there is anything else in the pocket or wherever you carry it, then use the protector. I always carry my device in its own pocket, so no risk of scratches. I have not a visible scratch on the Incredible 2 or my 2 year old OG Droid that has been dropped face down on the screen probably a dozen times on asphalt and concrete with no damage.
Gorilla Glass makes a huge difference compared to soda-lime glass, but still is glass, and as such, can break or scratch or chip if the circumstances are right.
