Screen protectors are like condoms. Sometimes they can reduce sensitivity, and sometimes they don't look pretty. But when something bad happens, and you didn't have protection, its your own fucking fault.
I mean go on Youtube and something and look at screen protector and gorilla glass tests. Gorilla Glass 1, 2, or 3 are scratch and impact resistant, not proof. Under the right conditions, something can and will go wrong, and you'd kick yourself in the ass if you could if you knew cheap sheet of plastic could have prevented any sort of permanent damage. And mentioning you've been using a screen protector since day 1 on eBay doesn't fall on deaf ears. People do take notice, and they pay $20-$70 more for a device THATS BEEN TAKEN CARE OF, than devices listed with scratches and scuffs here and there. So fucking what if your "Galaxy S2" has Gorilla Glass... There are newer devices with newer, denser, stronger versions of Gorilla Glass and they still can scratch by denser objects and materials. And if you're a baby, and complain about "Oh but it ruins the beauty-" No you dumb mother fucker, most screen protectors actually cover up and conceal light scratched with the benefit of protecting from more scratches. And this is even after talking about high-performance scratch-resistant GLASS... Don't get me started on the cheap plastic of the NDS and 3DS.
News flash, not all Nintendo DS and 3DS stylus come off the mold/manufacturing process perfectly. A small amount get past inspection with a tiny bead of plastic protruding out that you can't notice with your eyes, and they can and will cut deep into your cheap 3DS screen. You think a screen protector ruins the beauty of the screen? Well you know what else does? Scratches. Even if protectors add as little as 10% more protection, don't you thin that makes them worth it?
Screen protectors are something I'm very passionate about, because they're incredibly cheap and can prevent minor and major accidents alike, and preserve the value of your investment. Because here's the thing about a screen protector:
Screen protectors are temporary, scuffs and scratches are permanent.