If the vehicle rolls, all bets are off. A NASCAR style restraint system will keep the driver in place, but a normal one is not designed for it. While it's easy to come up with isolated instances where a person may have been better off without a seat/shoulder belt, the math supports wearing one.
I've engaged in a lot of hazardous activities in my life. Not wearing a seatbelt is kind of foolish in my opinion. It's cheap insurance.
"The first thing you need to know, will likely be the last thing you learn." (Unknown)