My point was that a prop is pretty easy to model as a blurred object, as it's relatively flat and can be done using alpha channels - a prop in a sim is usually 2 or 3 distinct models that are swapped out - a static prop, a slow moving blurred prop and a fast moving blurred prop.

On the other hand, an engine is a far more complex shape, and what shape does it make when it's blurred? Motion blur can be simulated, but it's heavy on the resources.