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I think that all your points are pertinent, however, operators will only equip their vehicles with aerodynamic devices according to what they see other operators doing.
The extra lamps are a fad (presumably dictated by the drivers).
An unladen truck will consume less power than a laden truck, therefore less fuel.
Operators are predominantly interested in the laden fuel consumption and will therefore ignore unladen figures (or factors that will affect it). Air deflectors could be made adjustable, but it's doubtful if the drivers would bother (after all they don't bother to cover the empty bed - which would reduce drag). Maybe the owner hasn't requested their drivers to do that.
Skirts would reduce drag, but cost money and restrict access for maintenance (and are liable to damage).
It's possible that a 'half-arsed' deflector might be almost as efficient as a full-height one (the angle encourages 'deflection' whereas the frontal are of a full-height one has to be pushed through the air (frontal area is a big factor in drag).
What you are seeing is the application of generalised solutions instead of tailored ones specific to each truck. Better than nothing in many cases, but customising the actual devices to the shape and size of whatever is being carried would be better, but time-consuming (and maybe expensive).
Try telling the drivers to sheet over their empty beds and see what response you get . . .
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