Sport: These small newly invented and world wide marketed quad copters are fine as long as they adhere to FAA Regs. and AMA suggestions, also known as rules but not stated that way.
Yes, they can endanger an airliner if injested into the engine of a Jet airliner or a small private jet engine.
Yes, they do present a danger to small propeller driver aircraft. Having one of these striking a perfectly balanced propeller, not counting, severing fuel lines , hydraulic hoses, electrical wirings, control surfaces, yes it could prove a real problem.

These small insects of a quadcopter resembling a drone, are one in the same. They fly by a ground Pilot, experienced or not, they do get flown by an idiot not abiding by the rules.Most of them are not AMA members, and no I can not states that as actual fact, nevertheless, they are the very people that will ruin this hobby, you can count on that!
Buying a new electric quad with a 5-15 year old at the local hobby shop it where the problem exists.
1. They know absoulty nothing about RC/ the AMA, or the written rules by the FAA in Print, and just go out and charge the battery, and let er rip.!!! They do not think about the hazards, Airliners, Small aircraft or Helicopters, that are in the air trying to perform a service, and " there is the danger". Like it or not, when these objects are in a flight path of a full scale aircraft, they do present a clear danger to the aircraft and people on board.
I don't do not think beating this thing to death will serve any purpose at all. I, and this is my opinion only,

Keep people educated on flying, know the rules, FAA and AMA Safety Code, and abide by them.
Keep advised on NOTAMS published daily by the FAA in Busy Airport Areas.
If we do not take any action on this, and the advisories sent to us, this hobby could someday become extinct.
No Flying Remote Controlled Aircraft, Period. Once at Fatal accident is proven, without a doubt, one of the quad copters created the problems, then it's over.