Paranoia, basically. I also treat devices/screens extremely well - to the point that I never use a case, am very picky about the surfaces I set it down on, and likewise have a dedicated phone-only pocket; yet I've twice encountered a mystery scratch eventually appear on the screen of previous devices, despite them having gorilla glass. I suspect a particularly hard stray particle made its way into my pocket somehow.
With the folding form factor of the Gemini and Cosmo, I was actually
more paranoid about those. When closed, the keys press hard against the screen, and rub ever so slightly, so if such a particle capable of scratching the screen were to make its way in between, I reckon it'd make a scratch a lot easier than just being in my pocket adjacent to a screen.
I kept the not-great-but-barely-adequate factory screen protectors on my Gemini and Cosmo for a long time. I eventually replaced the Gemini's one with a better one, but hadn't yet got around to it for the Cosmo. In both cases, the factory screen protector picked up significant scuff marks from the keys. I'm sure glass would be less inclined to scuff like this, but that's what I'd been trying to avoid impacting something that's costly to replace:
Hmm, I've never noticed a difference in sensitivity or friction. Perhaps that's to do with the particular screen protector. Initially, there is increased friction, but that's the case with a new screen as well. Rubbery or matte screen protectors don't feel good - I go for the shiny hard plastic ones.
Otherwise, I've heard from others that a screen protector provides a little shatter protection; but I've not had to put that to the test!
