No one uses?
Almost everyone applies a screen protector at purchase, whether they buy one to apply themselves or buy a Belkin in-store to have Apple Store staff apply it.
The rule of thumb is to never even touch the screen until a screen protector has been applied first. I've done that with every iPhone since the original, and I've owned all models since then.
Apple doesn't care what you do or don't do, it's called personal responsibility. You don't have to wash your car, either, but the automaker also won't cover it under warranty when your paint starts peeling off the car due to environmental damage. They also won't cover front end rock chips, which are prevented by applying the automotive equivalent of a screen protector, the clear bra.
Other manufacturers do it differently - Samsung applies a screen protector at the factory, which Apple could also do but they would have to increase prices to do so, and they prefer to treat their customers as adults and allow them to decide for themselves which protector best suits their needs, even if that's none at all.
It is a strong screen, especially when it comes to shatter resistance.
But strength does not mean scratch-resistance - however, even then something harder than Mohs 5.5 is required to scratch the screen, so most materials will not: