I hate using my phone "naked". I do have AC+ so I guess I shouldn't care but I do because I like to look after things I buy. Yeah, part of it comes down to keeping it in good condition for when it comes time to sell/trade it but I am also wanting to keep it in good condition for myself.
First thing I do with any new phone is put a screen protector and case on. Screen protector is an absolute must because it WILL get scratched and I don't want to feel the scratches when using my phone. Also the coating WILL wear down from oils on your fingers.
I paid £1,500 for my phone and yes it has insurance but spending another £20 for 6 screen protectors and a case will pay for itself. For me it already has paid for itself. I am reasonably careful with my phone but I have dropped it or had it slip out my pocket. I dropped my phone and the screen protector did its job and cracked instead of the screen. Swapped out the protector in a couple minutes. AC+ is there to use I know but the best kind of insurance is one you don't need to make a claim. It "only" cost £25 to fix a broken front glass panel but then I need to actually send it away or waste my time going to the Apple Store.
My case is a Spigen Liquid Air. It feels good, not bulky, and it's minimalistic. I can't bring myself to not use a case. There is no benefit to NOT using one except getting to see the frame/back panel. I don't know about anyone else but I spend most of my time looking at the screen of my phone.