DP is home, the following is his arguement for not wearing full leathers every day.
The major cause of death in motorcycle collisions are a broken neck or crush injuries. Leathers will not stop these two types of deaths and in many instances they will help cause it.
At approximately 72 degrees F the human mind functions optimally (although this varies in people used to significantly different climates). Every 3 degrees F above this figure you lose X percent of your processing/calculating ability. Imagine a summer day, the temperature when stood still in shorts, t-shirt and trainers is perhaps 80 degrees F. As you get hotter decisions become harder to make, stresses become less bearable and your thinking slows down. So on this 80 degree day you put your full protective leathers on and go out on your motorcycle.
In slow moving traffic or worse still stopped for lights/roadworks etc the temperature in leathers can easily exceed 110 degrees F. The things that malfuction at those temperatures include your vision, concentration, calmness and spatial awareness. These are the things that help to ensure your survival as a motorcyclist. Because there is no direct air flow to your skin when you do reach the open road you do not cool down much for the greater speed. Air vents in modern leathers/textiles help, but not massively. You may well open your visor to get a bit more air, but you are then liable to objects such as insects, ciggarette ends and gravel injuring your eyes. If not your eyes then the delicate skin of your face. You are now riding along, even without an eye or face injury, without an accurate judgement of the distance of the vehicles around you or of potentially hazardous junctions/roundabouts or road surfaces.
(The onset of this malfunctioning at high temperatures is not dramatic or noticed, it creeps up on you in the same way that impaired thinking in low temperatures does).
You may suddenly find yourself too close to the vehicle in front when it's brake lights appear and unable to stop. You may go to overtake the vehicle in front of you and find you have miscalculated the approaching distance and speed of an oncomming vehicle. This is bad judgement brought about by overheating.
I would prefer to ride my motorcycle without protective leathers/textile gear as I consider myself to be more in control of my motorcycle than I would be if I were wearing full leathers in hot weather.
The increase in risk caused by getting something wrong while wearing leathers is an increased risk of death, paralysis or serious injury. The increase in risk to me by wearing shorts is an increased risk of abrasion but a lowered risk of rider-error (the largest attributable factor in motorcycle collisions).
I am aware that I may be knocked off my motorcycle by a reckless or careless driver but this has happened to me twice in 30 years of riding. I ride up to 30,000 miles a year (the average for a biker is 5,000). Thus I consider that the increased risk of rider error caused by wearing overheating leathers is more than balanced by the lowered risk of not getting into an accident in the first place.
It is a common misconception that a bikers leathers protect from most injuries. In fact the sole injuries they protect one from are abrasion and occasionally (if the suit is well armoured) a broken elbow/kneecap. I have never in my riding history heard of any biker injured beyong repair by abrasion.