Every phone will show its wear and tear, regardless if there is a case and/or screen protector on it. If somehow you manage to have that phone come out flawless after say a year's worth of use, that's pretty amazing, even with a high quality case and screen protector. Fact is, these phones take daily abuse with the occasional drop or two. I've NEVER sold a phone that was flawless, but the main thing is the screen and as long as that's flawless people pay a premium. So yes a screen protector would be a wise investment if you plan to resell your old device, but no matter how high quality the protector, it changes the feel of the phone, period.
I know many who refuse to use one just because it changes the feel of the phone. The feel of glass underneath your thumb or whatever finger you use to navigate is much easier to navigate with compared to any other phone with a screen protector. The gamble is you risk small scratches and such from normal wear and tear. Considering how much the average person uses their cell phone, a small scratch is pretty much guaranteed as well during your time of ownership, even in the case with your iPhone in its protective pouch.
So yes I agree that a small $8-$15 investment in a screen protector is a good way to keep your shiny new device as clean as possible. However, I am not the user of anyone else's phone but my own, and I can't tell them to like the feel of a screen protector either. It's definitely something to get used to and it's not for everyone.
Unfortunately, for the prices we pay for these devices we expect them to take quite the beating. It also doesn't help that many people tend to mix up the meaning of "scratch resistant" and "scratch proof", where as no phone is scratch proof, but some are more scratch resistant. These simple marketing ploys give us a false sense of assurance that these phones can take a beating (which they can to an extent), but not as far as we imagine they can. Fact of the matter is, these phones go through daily abuse and also that the face of the phone is still glass. No matter how strong the glass a scratch is still possible because it is still glass. There's a reason why no one demos their phone rubbing sand into the screen. (Sand will easily scratch your smartphone's screen for those who don't know).
So I always recommend a screen protector, but for those who choose not to use one, don't expect your screen to be flawless at the end of your term of ownership.