Like my bicycle helmet, I use them all the time. I recall as a kid seeing a 50's four door sedan get t-boned and the seat and driver came right out the other side. I feel secure with the belts and naked without. Another thing I do when taxiing the Yak trainer I keep my shoulder harness done up until the plane is stopped and the prop is no longer turning. I have heard of pilots undo their shoulder harness and for some very weird reason the plane either hits a bump or hole and flips on its back resulting in a crushed head.
