[aerohead]

Some articles addressed pickup truck tailgate spoilers in psychological terms.
The automaker ( Ford ) has understood, since 1981, that a pickup ( their's anyway ) will typically get better mpg and lower rear lift if the tailgate remains upright and closed.
Ford, presuming that consumers have some limited understanding of rear spoilers, by placing a spoiler on the tailgate top, are hoping that, when not carrying long loads, owners will close the tailgate in order to 'preserve' the function of the spoiler.
It's not that the spoiler itself necessarily reduces drag, it's just that its presence, somehow encourages the owner to keep the gate closed, which absolutely does reduce drag.
In the early 1990s, the now defunct, Sport Truck Magazine, devoted a major portion of one issue to pickup aerodynamics. Aerodynamicists from all the major automakers contributed to the article. At that time they recognized aerodynamic ideosyncracies among pickups, and the reader was cautioned to approach pickup aero on a case-specific basis.
Today's ( over-bumper ) tailgate spoilers take better advantage of a truck's overall length, improving fineness ratio and flow reattachment area potential. It would be surprising to learn that they did not help.