Lap belts keep you in the car. Three point and up keep you from being injured or killed.
It's all about risk assessment, at least for me. The chances of flipping over in an open car are very low. The chances of a side impact are moderate. The chances of the front of your car striking something and you slamming into the dash/steering wheel are very high. Honestly there is no way to predict what will happen or if you'll survive. I say prepare for the most likely event: a direct or glancing blow to the front or getting rear-ended. Shoulder belts combined with a lap belt will probably let you walk away, or maybe with only minor injuries.
I'm lucky to have walked away from a 60 mph smash into a guardrail wearing only a lap belt, in a Spyder. The only thing that saved me from smashing directly into the steering wheel was the spin that happened from hitting the guard rail. I have a permanent loud click in my neck from severe whiplash. It took me several months to recover from the back and neck injuries and return to work.
I now have 5-point Race Quip harnesses in my NEW Spyder. I put all five points on, every time I go for a ride. It is easy, and you get used to it quickly. They also have a twist quick-release. Each belt snaps into its own hole in the buckle/release.
The buckle/release is on the right lap belt. So you get in, snap in the crotch belt, then the left lap. The the two shoulder belts one at a time. It takes maybe 10 seconds.
The side benefit is they keep you planted in the seat, not wasting energy and attention on your position, which lets you focus on being a better driver.