If tethered to the bike, then it does look like it would help. However, if the rider decides that he's gonna make a quick trip to the corner store and forgets to tie himself to the bike, then the airbag would only make him fly higher up. Especially on the foreward-lean sport bikes. Now, if the guy t-boned a big SUV or something, then i guess the airbag would help him clear the vehicle and avoid a broken neck.
Also, like 1percenter said, if the airbag inflates prematurely, then that would really mean trouble for the rider.
Maybe if we equiped every rider with parachutes and the bikes with ejection seats... Now there's an idea!