[jmercouris]

Turbulence that results in people flying out of their seats is exceedingly rare, like a once in a lifetime experience for a commercial pilot. Airplanes will go to great lengths to avoid bad weather to avoid stressing the airframe and making passengers uncomfortable.

[borski]

Anecdotally, I've been on more planes with lots of turbulence (that without the seat belt would have been potentially damaging) than I can count on two hands. I've never been in a plane crash. :)

I imagine part of the reason people don't fly out of their seats today is, well, seatbelts, but also likely depends on the airline/pilots as well.