In my accident, the wellnut setup did not keep the rear portion of the body attached to the frame. I was going about 35mph. I wasn't wearing belts and lifted up maybe 6" off the seat. If I had been wearing belts, I think my weight being straped to the body would have possibly caused the body to come completely off the chassis.
There are so many variables to this question and so many different factors to consider. The main one being speed. Hey, I was driving careful and looking out and I still got hit.
Now, when I used to get on the highway, I wore belts. I don't even know why. Maybe because I felt safe. In my opinion, at high speed, it wouldn't matter that much whether you wore them or not. Looking at the pic of that accident, it looks like the guy may have been better of not being attached to the body. But EVERY accident is different.
When I get going again, I won't wear them just cruising around town at fairly low speeds. I'll wear them when I get on the freeway, but I don't think they would make a difference.
Bottomline, at highway speeds, I guess it depends on what kind of funeral you'd like. Open or closed casket.