Well I don't know what kind of pants you guys are wearing or what your doing with your phones. But after an entire year of owning my previous iPhone, keeping it my pocket naked, and heavy use I can say I have zero scratches on the screen. The silver bezel around the screen is covered in dings and scratches and looks pretty bad and some of the corners and edges have dings and marks from hitting pavement. I've dropped it several times, a few on cement and asfault (landing on the screen sometimes) and it still has no scratches on the glass. Maybe I'm just lucky, but normally I am not.
No protector will damage the screen that I know of. And the change in sensitivity is very small, but enough to where I didn't like it. But in the end its up to you, I know I was paranoid when I first got my $600 iPhone last year but after awhile and seeing the videos online of the beating it can take I changed my mind. Plus its nearly a crime to cover up something that looks so good.