I notice you can use paddle shifts on steering wheel even while in D mode- the dash indicates that the tranny goes back to D mode after down shifting on paddles...............Is there a reason NOT to assist brakes and paddle shift down even while in regular D mode?

 

The ability to overrride the auto shifts adds a driver interraction feature . "Drive" will shift the car but it can't anticipate abrupt changes as far in advance as a driver . On the other hand "drive" presents an option for a driver who is not certain of ideal shift points .

 

Cayenne is like this too

 

and the majority of AUDI tiptronics.

 

Probably cheaper to put the wear on the brakes, although in the sportier modes, PDK will downshift depending on how you drive.