I see no reason to replace them both unless the hub was trashed because of failed bearings.
Or maybe it is just a marketing scheme to get you to spend a few more bucks for something you don't need, but all the same it is nice to not have to bust the old ones apart and reassemble them.
I did go with the loaded calipers for my J20 though. Makes brake work so much easier. Not to mention how much better they worked over reusing old crap last time around. Smooth as silk and they have never been like that before.
1981 J-20 "RED" 360/727/208,R.I.P. Undergoing major retrofit to be a 1980 Utility truck.
1977 Honcho "Blue" 401/400/quad. Under going major upgrades.."This is Not Your Fathers Honcho", it may not even be a Honcho anymore when done.