I think it depends on the camera and coverage. But I'd rather have a bigger, more useful rear windscreen than a back-up camera.

High end cars have blind spot detectors, but any car sold in western markets must come equipped from the factory with side mirrors. It's the law.

Yes, it would be better for fuel economy if we could have side cameras installed on cars, trucks and buses instead of mirrors, but again... legal restrictions prevent this from happening.