The whole point was with "rear section is already shaped and works like a diffuser (gradual rising slope)" to have more than just a "void" as the Diffuser. The tape merely holds the rear most few inches against the fascia, the rest of the big fin is very firmly secured to the panels ahead of the fascia. It's not going anywhere, and hasn't. I'm not saying it gives any great performance advantage, but it doesn't do nothing. A louver flows more air than a hole.

And yes it is more for aesthetics than function, though it does function, the longitudinal fins (in any diffuser) are there to straighten the airflow, and yes you want them as long as possible but on road cars they don't extend forward of the rear axle. These fins are as long as those on an OE diffuser on a Viper. Look at pictures of other diffusers such as http://www.roeracing.com/productcart...154_detail.jpg

The Mopar diffuser (described by them as an appearance accessory) just hangs stubs of material at the very end of the "rear section is already shaped and works like a diffuser" that are about an inch and a half wide, stick down a few inches and extend toward the front of the no further than the fascia.