The following is purely MY opinion and I’m not trying to coerce anyone into believing one way or the other. I don’t drive on the island but then, I don’t drive at home if I can avoid it.
However, my dear husband does and has never found the driving to be SCARY, not even back in the 70’s and 80’s when too many of the roads were more like paths than anything we would consider to be a real road.
We’re more scared of the driver who has imbibed too freely or the person who can’t let their cell phone go unanswered and particularly, the suicyclists doing their craziness, but that’s us. And those things aren’t scary - IMHO - those things are just plain craziness!
As to whether or NOT one needs to rent a car, to me that is a purely personal choice. If you want to get out and really explore all the island has to offer, then YES, you need to rent one, even if only for a few days. Having a rental gives you a freedom you would otherwise miss out on.
But there are island buses that service the popular locations and certainly taxis will take you wherever whenever but for us, that requires planning and waiting and very little freedom for decisions made in the moment.
There are many vacationers who want an AI resort type vacation where they can lie on the beach and be catered to without having to think about anything, including where their next meal or drink might be coming from and there’s nothing wrong with that. And for these people, a rental car would probably be a total waste of money. But that certainly wouldn’t work for us.