So lets talk about B)
One of my friends spends a lot of time on race circuits in the 'fast' group, he loves track days, Californian Superbike School, IAM and RoSPA Gold etc....
Anyways, he said Steve, if they are so good, why don't Superbike Racers wear them, or racers at the TT... why don't they wear them.
In my opinion... it's all because of their environment.
On a race circuit, they are designed for large run-off areas where you can slide for a long way before hitting anything. Unless you are VERY unlucky and hit another bike at speed, or high side (very high), you shouldn't experience a 'sudden stop', and it's those 'sudden stops' that do the damage (as testified in the earlier post in this thread, 10mph landed on his chin and broke his neck). On the TT they are feckin nuts, and that's part of the deal. At the speeds they do, and risks they take a neck brace is not going to save them.
Horse riders are prone to this injury, slow speed, bucked off horse head first onto the ground (all slow speed, but a sudden stop on the neck). They wear Helite Airbags now as this situation is infrequent so suits this 'one time use' application.
Motocross racers on dirt, jumping over crests, inbetween trees and rocks. They wear neck braces now as frequent crashes suit this application.
Road Riding.... with street furniture, curb stones, walls, fences, traffic. There are lots of opportunities for a 'sudden stop' in the event of a high side, low side or crash. But unlike a horse (with a mind of it's own) a motorbike generally does what you tell it to, so most riders think 'it'll never happen to me, I'm better than that' .... which might be true, but you can't control everything and one day when you do need it... it won't be there
Off-Road riding, hey YOU ARE GOING TO FALL OFF guaranteed haha... Get one.
Don't see many trees and rocks on MX courses - generally open areas for racing
Enduro terrain yes, haven't seen many neck braces on Enduro events