I don't personally agree with them, there are cases where not wearing them save you. Mt brother was tossed out of the windshield instead of getting crumpled into a small box of metal that was once a car.
Motorcyclists would be hitching a ride on a death trap if they were strapped, whereas, even at 50+ you can, if practiced, sort of roll out of a fall and you just slide on the pavement. You may get a little banged up but rarely a trip to the hospital. Impact another car or object and you are probably screwed, but I would say no more screwed than being in a car and getting tossed out the window at 50+mph.
I always felt it my decision to handle my life how I desire, so if I want to die and don't care, I can not wear my seatbelt, if they catch me, I am happy to pay the fine.
I have since long ago just started wearing it as every car I get into I get told to put it on, so it has just become habit. But I hate the way they ride on my neck, tighten on my stomach, and are generally restricting and uncomfortable. I spend more time adjusting with my eyes off the road than is probably safe, and this can't be don't pre-driving as belts tend to creep up on my like a loose pair of underwear :)