This is getting a bit off topic, but no, motorcycles would not benefit from seatbelts. They would actually be a huge detriment. You're right with the idea that it is advantageous to be separated from the motorcycle in a crash. Not just due to not wanting to be dragged along however.

When I started riding a motorcycle several years back I took a proper safety and rider training course. One of the strategies that could be employed for an imminent crash was literally referred to as a "Superman Dive". Basically, you see you're about to hit a car/truck so you put your feet on the footpegs, hands on the gas tank and jump over the vehicle you're about to hit and then do your best to roll when you hit the ground on the other side. A seatbelt would naturally prevent this.

While hitting the ground on the other side of the vehicle and then going for a roll is far from a good day, it beats going from whatever-speed-you're-at to zero instantly when you're flung into the other vehicle. Since there is no metal box around you on a motorcycle, rather than be a safety feature, a seatbelt would limit or prevent actions that could seriously reduce the chances and severity of injuries.