Second, part of the purpose of the seatbelt is to keep you in the driving position for the entire duration of the crash so that you can maintain as much control of your vehicle as possible until it comes to a complete stop. If the first impact of a crash throws you out of the position required to drive the car, you're no longer capable of controlling the vehicle in such a way that reduces the severity of or prevents further impacts. As some of these further impacts stand to involve persons not involved in the initial impact, it follows that wearing a seatbelt stands to protect them as well as you.
Third, We are in absolute agreement that DUI should be treated more seriously.