In many states, seat belts (believe it or not) are not required for anyone but the driver and front seat passenger. Sideways seat belts carry a different risk of injury than those in forward facing seats, but they do keep the occupants from flying around the inside of the vehicle in a collision; the most common cause of accident injury and deaths.
I'd think that the sideways lap belts would be better than nothing.
AIR 2053 Current: 2006 Born Free 32 RQ Kodiak Chassis, & 1995 Coachmen B-van
Former Airstreams: 1953 Flying Cloud, 1957 Overlander, 1961 Bambi, 1970 Safari Special, 1978 Argosy Minuet, 1985 325 Moho, 1994 Limited 34' Two-door, 1994 B190 "B-Van"