There's a whole real-world project devoted to developing ornithopters.

Here is a list of (their) reasons why an ornithopter is a good idea.

1. Improved Efficiency. An airplane propeller is only about 70% efficient. Energy is wasted because some of the aerodynamic force produced on the blade acts to resist the motion of the propeller. In an ornithopter, the downstroke blade resistance provides lift, and we can feather the blade during the upstroke so resistance is minimized. Therefore the ornithopter has potentially higher efficiency than an aircraft with a rotating propeller.

2. More Lift. Flapping wings have some additional ways of producing lift and thrust that aren't available with fixed or rotating airfoils. One is the clap-fling technique, first discovered in insects. By bringing together the wings and then abruptly flinging them apart, a powerful burst of thrust can be produced. Another technique is delayed stall. Flapping wings don't stall as easily as fixed wings, because the cyclical motion doesn't allow much time for a stall to develop. In some situations, it might actually be useful to stall the wings, because the downstroke air "resistance" is actually a strong lifting force. These techniques can be used to improve the slow flight and hovering capabilities of the ornithopter.

3. High Manuverability. Whereas an airplane relies on its forward speed to produce maneuvering forces, the flapping wing can produce large maneuvering forces at any time. The incredible maneuvering of birds is partly due to their small size and partly due to their use of flapping wings.

4. Reduced Noise. Airplanes and helicopters make a lot of noise. Much of the noise is produced by the rotating blades and not by the engine. High noise levels are one factor preventing wider use of the helicopter. Adalbert Schmid reported low noise levels for his manned, engine-powered, ornithopter flights of the 1940s. Radio-controlled ornithopters are also fairly quiet.

Here is one of their flights
As you can see they haven't gotten control worked out yet, but it did get off the ground.

So if the people of Dune have figured out how to get Ornithopters working properly, good for them!