I will not fly in an aircraft that does not have a shoulder harness, full stop. With respect to helmets, I only wear a helmet in conjunction with a parachute in an aircraft where it is realistically possible to bail out of. The advantage of the helmet is to protect you head if you hit part of the aircraft as you are bailing out or hit something on the landing, especially if you are landing in trees.
A bit off topic but if you want to wear additional safety equipment in a GA aircraft then I strongly recommend a pair of nomex gloves. If there is a cockpit fire or you have to vacate a burning aircraft on the ground the ability to grasp something very hot may well be the deference between surviving...... or not.