SO you've been lucky so far. OTOH, I've never had one scratched (or cracked) either and I'm not at all bothered by a screen protector. So, what have I got to lose other than a few dollars? They may also provide just a bit of help if you're not using a case, too. Buy a good one, put it on right, it does not affect the phone in any material fashion that I see. The contrast and brightness blows right through it when the screen is on. (That was not the case in Palm Treo days!) Maybe I'd feel differently if I sat there doing A-B comparisons of varying protectors and naked screens. But if I have to do that to be concerned about it, it's already doing just fine.
As we evolve to keeping pricey phones longer, it is ever more likely something goes wrong. Murphy's law.