gorilla glass and other are good enough where i can trust that they wont scratch. There's nothing I hate more than seeing my friends screens covered up with horrible screen protectors. Completely ruins the phone IMO. As beautiful as my SAMOLED screen is, a lesser display without a screen cover looks better than that with a cover.
I also feel the same way about huge ballistic cases. Why buy a slim sleek phone if your going to have a huge case. It takes away from what the phone is intended to be.
We are going to have to disagree. I'm not taking a chance with a ~$500 device. Gorilla glass might be better than regular glass, but my experience with standard tempered glass is that if you put it in a bag with a bunch of sand from a florida beach and shake it for a while, and while you won't have real scratches, it won't look as good as it did before you put it in. I'm not taking a chance...
My wife doesn't use a case or any kind of screen protector and she throws her phone in her purse and I can tell you that they look trashed in a matter of months. By the end of a year, they look like someone hauled them through a mission in Iraq. The screens are scratched, the black plate is worn, the metal bezel is covered in scratches and sometimes even the carrier logo is half worn away.
You can think that I'm crazy or wasting money, but I'll keep buying screen protectors and using cases, and some day when my wife pulls a phone out of her purse after ~6 moths of jangling around in there and she shows it to me and looks brand new, then I'll rethink my strategy.

As far as the screen protector affecting the image quality, I'm a bit obsessive about these things and I can tell you that if you saw my phone, you'd have a hard time knowing that I have a screen protector on it.