Well, on Desperado, I felt like the lap bar wouldn't have helped me a bit. It only came up to my mid-thighs, but the seatbelt held me in as tight as I wanted. All the lap bar ended up doing was giving me bruises. So I guess that for tall people (I'm 6'1", BTW), the seatbelt can help hold you in more. Some of the smaller guests I saw had the lap bar touching their chest, so that could be one possible explaination on this ride.
Overall, though, I'd be inclined to go with the redundancy theory of Jeff's: If on restraint fails, then the other can back it up.
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If bees make honey, then do earwigs make chutney?