The only real use is as expansion of capacity for broadcast recordings.
Panasonic have mentioned in the past as it being a backup of material, but it cannot function in reality as a backup because if the machine fails, the material on external HDD cannot be accessed on any other machine.
I dare say the thinking with AVCHD material is that the only reason you would move it onto such a machine is to use it to archive the material to Bluray.
For editing on a PC they have other solutions to work in conjunction with the AVCHD source ( i.e. camcorders) such as HD Writer AE software for example.