I used to wear nothing when in a tree stand. Then I got smarter and bought a safety belt. It was better than nothing and it saved me from a 25 foot fall, which would have most likely killed me. I only fell 3 feet and when the belt stopped me, it slid up onto my lower rib cage and broke a rib. So there I am, hanging 25 feet up, almost dark, with a broken rib and the safety belt tight as heck pushing right on that broken rib. Was a long slow climb down and walk back to the truck.
So after that I bought a full body harness. I feel much safer and I don't even know I have it on.