I never buy any insurance when renting.
When they try to convince me, I tell them I already pay monthly premiums to cover myself in case of an accident.
They counter that I should buy to avoid having to make a claim on my insurance and have my premiums go up.
I say back, that basically they want me to buy insurance on my insurance. I pay premiums in case I happen to crash. I don't plan on crashing, and assume that I won't. However, on the odd chance that I do, I want to/have to be covered. So basically they are advocating that I pay even more now, to avoid the eventuality of needing to come on to my insurance!!!!
With that argument, I should always have two different policies on my personal car!!
The same way I am comfortable driving my personal car 365 days a year, even though if I crash my premiums will go up, I am just as comfortable driving a rental car for the weekend knowing that if I crash my premiums will go up!
It's the most ridiculous argument to advocate for buying insurance 'just in case', to make sure you're 'covered'.