Why the "Fan Overtime" feature sound like bad thermal dissipation due to that "Dust Defender". I reckon that to achieve any cooling benefits from any sort of cover over the circuitry you would need to create a laminar flow between the PCB and the cover. That would require a very strong and noisy fan and a smooth surface of the circuitry so that no air turbulence is created.
Imagine a CPU fan with uneven, rough and dimpled blades, that would be really inefficient.
I'm no pro in this field but this seems logical if I'm not missing something.