well i know for the Audi TT.
the first cars didn't have the rear spoiler.
and they had many accidents in europe with the rear being loose at higher speeds.
and by higher i mean over 60 mph.
till Audi eventually acknowledged the design flaw and fitted the rear spoiler that is now on all TTs.
at least with the crossfire when you are parked you don't have this huge wing up.
also the boxster has it,
so does the 911, SLR etc
i am sure the porsche designers knew what they were doing.
i wish ours was like the SLR.
if you brake hard the rear wind deploys at 90 digress to the bodywork and acts as an air brake.