I don't think you can count on that animation as being an accurate representation of a wind tunnel smoke trace.
That said, Toyota has to take rear lift into consideration. While nobody here is likely to go blasting down the highway at top speed in a Prius,
others apparently will and do. So the company needs to ensure the car is stable at high speed, even if it means sacrificing ultimate aerodynamics that would be safe at, say 60 mph.
Not saying that's unquestionably what the Prius spoiler is doing. It could also be reducing drag. (See the item about the "upkick" in the Volt's rear spoiler reducing drag, here:
http://ecomodder.com/forum/showthrea...ures-4629.html )