Ian K said..
Much easier for the faster vessel to forward scan than for the half-speed vessel to be fully aware of what's going on astern.
Good point. But in many spots, room to change course is quite limited. At our local spot, we have to pinch hard just to make it past the swimming area. If there is a kiter (or winger) before us, there's no way to keep a course that's 15 m upwind or downwind. Neither is jibing an option, since you'd end up in the swim zone. Quite annoying when a kiter in front of you just decides to turn on the spot, without looking back.
In the incident described by nauli, part of the problem is the faster speed of the speed surfer. If he's going at 30 knots, he is gaining 10 meters per second on the winger. If the winger does check behind him 5 seconds before he turns, the windsurfer is still 50+ m behind, which may look far enough. If the windsurfer is on a speed run going downwind, the winger would have to look 180 degrees behind him, which is hard and unlikely to happen.
Chances are that the winger in this incident did not look, but speed differences (and different preferences about going in a straight line) make things a lot worse. Another winger moving at the same (slow) speed as the first winger would have had enough time to change course.
Bottom line is that physical separation makes a lot of sense. At our windsurfing beach, many kiters and wingers use the same narrow launch, but then go upwind right away to get into clear waters. When their skills and the wind are good enough, they are never a problem. Problems only arise when skills or wind are marginal, or with the occasional kiter who insists on sailing always in the same "lane" as the windsurfers.