In my opinion punching air has it's place in allowing you to work on your technique and form but doesn't replace hitting pads or a heavy bag. If you punch air and air alone you will be surprised the time you have to hit something with substance with any power. If your lucky a sprained wrist will be all that you get. Punching bags is totally different than air and actually hitting a body is different than the bags. All should be practiced to some extent.