I would much rather have the park assist on the front end rather than on the rear bumper. If you have the nav, you can just use the camera to see if you're about to back into something, but the sensors in front would be helpful when pulling into parking spots and such.

 

Backing up towards a non-descript (ie. poorly lit gray concrete) wall with the rear camera does you no good. You have no sense of depth. With PA, I can get within 3-4 inches.