I agree that the use of quad copters is becoming a threat - when used without common sense...
I do fly for a living and I'm also a keen RC plane geek...
Recently, whilst flying my rc plane, a guy came over with a quadcopter. I got talking to him whilst he was using his quadcopter and he told me that he held an "online record" for having reached certain speeds and altitudes with his equipment - which he can prove with onboard GPS... His "record" was 2800ft...
I was alarmed as where he was flying was in the Northwest of the UK with very busy airspace overhead... Liverpool arrivals at 2000ft, the Low level corridor overhead with light aircraft at max alt of 1200ft or so and Manchester departures also overhead... He was unaware of all of this as he was just "a guy of the street that went and bought a quad copter"
I'm all for the hobby of rc flying but the invention of being able to fly out of sight of the operator has its implications...