I've flown over a quarter of a million miles, and I've always kept my seatbelt on, even though I've never experienced the level of turbulence that would cause people to be thrown against the overhead. Why passengers don't stay belted is a mystery to me, as turbulence can strike at any time without warning. People who fly unbelted reveal themselves as rank amateur flyers. There's apparently too many of them these days, which is another reason I don't fly any more.
My sister experienced the worst turbulence that happened to someone I know. In the mid-1960s, her high school band from San Diego won a contest that earned them a halftime performance at an NFL game in Miami. They crammed all 100 or so kids and a couple of chaperones into a charter DC-7, and flew them at a relatively low, turbulence-likely altitude. The cabin crew first fed the kids, but they hit turbulence shortly thereafter. It didn't let up as they crossed New Mexico and Texas. The kids started throwing up, throwing up, and then throwing up some more. One boy sitting near my sister filled up three barf bags. Somewhere over Texas, they ran out of barf bags. Subsequently, the cabin became strewn with vomit. It was like something out of a Monty Python sketch! Except it wasn't particularly funny at the time to those involved. The plane had to make an emergency landing in New Orleans, where some unfortunate ground crew personnel spent a couple of hours cleaning the cabin. Interestingly, all the school officials and teachers going to the game took a regularly scheduled non-stop flight on a 707. They flew in high-altitude turbulence-free comfort, of course!