I am always amazed to hear the sound of seat belts unclipping soon after take-off. Some people just don't seem to learn, that cruising can be dangerous, until they get injured...
I must plead guilty to being one of those who frequently unclip their seatbely as soon as the sign goes off. The recommendation is usually to have the belt tightly fastened during takeoff, landing and turbulence, and to keep it loosely fastened at other times.
A loosely-fastened bely offers reasonable protection, permits movement within the seat space, and is much more comfortable on a long flight than a tight one.
With most belts, the obvious way to change the state from tight to loose is to unclip, move the buckle and then reclip. This is what I generally do, and will continue to do until inertia reel belts, or some similar technology appears.
I cannot, of course, answer for everyone, but I am sure that I am not alone in my mode of seat belt use.