I can say with 100% certainty that having a spotter scanning the airspace happens more frequently then you may think. During a sailplane flight the timers job is not only to give the pilot time updates but to scan the sky for signs of lift usually in the form of wind change, cloud formations, birds, other sailplanes but most importantly since an altitude of 2,000' is not uncommon, manned aircraft. During a Pattern flight, the callers job is not only to remind the pilot of the sequence of maneuvers but to be scanning the path of the airplane to avoid other participants and help with aerobatic box positioning as well.

Since I know that some in this thread are " show me the numbers " kind of guys. Perhaps he can show statistics where see and avoid has failed in keeping traditional models and full scale aircraft from colliding.