So, help me understand this: if parking brake is on, and transmission is in "Park", the transmission park detent is engaged. So, if the car gets rear-ended, the transmission is messed up anyway.
You could engage the parking brake BEFORE sliding the transmission in park, that would make more sense. But, the shock of a rear end collision will overcome the parking brake and still mess up the transmission park detent ...
Unless you keep your transmission in Neutral, and apply the parking brake ... and then you might not be able to retrieve the ignition key ...
So where is the advantage of a parking brake ?