Although a T-shirt or a tank top may seem like a good idea on very hot day, they will offer no protection in the event the rider is unexpectedly separated from his/ her motorcycle (fall). It may look cool but in actuality a rider wearing a T-shirt runs the risk of becoming dehydrated or sunburned, not to mention the possibility of road-rash. Save the T-shirt/ tank top for the car or the beach!
A denim jacket is a step up from a T-shirt, but just barely! At slower speeds (stop and go traffic) this type of jacket will give the rider a minimum amount of protection from abrasion. As the speeds increase (highway speeds) however, that protection is reduced significantly. The denim jacket will be torn to shreds if the rider were to fall at highway speeds!