With my limited understanding of aerodynamic principles, it seems pretty clear that a wagon without any rear spoilers and a smooth curved tailgate will create a low pressure zone at the rear. I can understand the purpose of the roofline upper spoiler, to detach the airflow more cleanly off the roof rather than allowing it to tumble down turbulently over the tailgate. What I can't see is what the purpose of the midline spoiler is.

Is it that the top spoiler isn't doing its job properly and this 'catches' the remaining airflow and channels it out horizontally? Is it just a handy mantelpiece for putting carriage clocks, birthday cards and china dolls on? Is it just cosmetic to imitate a Fiesta 'Flight' (or whatever those Fiestas were with those hideous midline spoilers)?

 

AFAIK separated flow from the top spoiler re-attaches onto the midline one before finally joining the flow down stream of the car, so it's really the equivalent of the rear spoiler that our unfortunate 4-door cousins use.