Unalerted see and avoid isn't "acceptably safe" for a lot of operations, hence the radio requirements at uncontrolled certified, registered or military aerodromes. That covers a very large proportion of the uncontrolled aerodromes, and virtually all that the higher speed Challengers, Dash 8s, Fokkers etc will be flying into. This is effectively separating the Fokker from the virtually invisible gyrocopter doing 70 kts even though both are OCTA and the gyrocopter might not be carrying or using a radio.