Yes, it is a big deal. In an emergency, I’d rather not have him tossed into my seat belted lap.
I agree. People who don’t keep their seat belt on are morons. I have been on planes that suddenly and with no warning dropped a 1000 feet. If you are not held down by your seat belt you will go flying around the cabin. I don’t give a damn about the moron who refused to wear his seat belt, but I do care about the innocent person he hurts when he lands on their lap.