As recent events in the news indicate, when you are on an airplane you should always keep your seat belt fastened. If you are not wearing your seat belt and unexpected turbulence is encountered, you basically end up getting thrown on the ceiling of the airplane. And then you get thrown back down on the seat/floor. While the distance aren't great, the speed can be very fast and there are a lot of hard, awkwardly shaped things you can hit that can do a lot of damage. Why take the risk of this being you? Just wear your seatbelt, it doesn't even have to be that tight, and you'll be a lot safer.