The two times I've rented a car in the USA back when I drove, I turned down all extra insurance also, figured that *if* I got the car damaged, I'd cover it. Haven't needed to an either occasion.
lolno
I do for my critical PC parts (CPU, GPU, MOBO) because I've had 3 GPU upgrades due to this in the past when they would end up dying after about 5 years.
I also got it for my home A/C which really came in handy for a while when I had a series of problems happen year after year. Sadly that ran out so all of the visits and parts that used to be free would now cost me.