The question is on my mind, because many passengers think there is no need to wear the seatbelt when the seatbelt sign is not illuminated.
The following is a list of deadly turbulence accidents on jet airliners since 1980.
Excluded are accidents involving jet airliners designed or built in the former Soviet Union.
For a more complete list see aviation-safety.net.
10 May 1980, Indian Airlines B737-200, near Rampur Hat/India:
The aircraft experienced severe en route turbulence. 2 of the 132 passengers were killed.
7 October 1981, NLM CityHopper F28-4000, Moerdijk/Netherlands:
Shortly after takeoff, the aircraft entered an area of severe thunderstorm activity. The aircraft apparently had a catastrophic in flight structural failure due to severe turbulence because it was seen to emerge from the clouds with one of its wings broken away. All 4 crew members and 13 passengers were killed.
16 August 1982, China Airlines B747, near Hong Kong:
The aircraft encountered severe in flight turbulence. 2 (out of 292) passengers not wearing seatbelts were killed.
3 October 1990, Eastern Air Lines Flight 791, DC9-31, Florida/USA:
The aircraft clear air turbulence at 31,000 feet (9450 m). 1 injured passenger died 3 weeks later.
5 September 1996, Air France B747-400, near Ouagadougou/Burkina Faso:
Severe turbulence associated with a weather front seriously injured 3 of the 206 passengers. 1 of the 3 passengers later died of injuries caused by an in flight entertainment screen.
28 December 1997, United Airlines Flight 826, B747-100, over Pacific Ocean near Japan:
The aircraft encountered severe turbulence during cruise about 2 hours after departing Japan. 1 of the 346 passengers was killed and 3 crew members sustained serious injuries.