Everybody has their own theories on this, but here are mine. I don't have seat belts in any of my roadsters. If it flips, I want as far away from it as I can get, don't want to be strapped in an upside down roadster. A young lady named Megan flipped her T bucket at about 70 mph when something locked up. I had a picture of it with the seating area smashed into a guard rail. Luckily she was thrown clear into the woods, and although she had some serious injuries, she survived. I think if she were strapped into it the outcome would have been different.
I also don't want a roll bar because of one not fitting the look of my cars. My '27 had a roll bar, but it was just a chromed one that was below my head level for looks.
The most important thing is to drive defensively, like you would on a motorcycle. Keep a constant eye on everyone around you (which is easy in an open car) and leave lots of space and drive conservatively.
What I do may not be the most responsible way to do it, but I think it makes my driving as safe as possible without going to the lengths of encapsulating myself in a roll cage with 5 point harnesses. By their nature our cars are not as "safe" as a new BMW with crumple zones, side air bags, etc. but our driving habits can help offset those deficiencies somewhat.
Don