The Multiplex RX switches off or isolates the battery pack that is deemed unusable and switches onto the back-up battery pack? Is the ill battery pack completely disconnected internally from the power buss? If so WOW...

Why not just use two batteries and switches as others have mentioned. Cheap, simple, perceived failures are the lowest of any other system... AND it works flawlessly... The perceived drawbacks are negligible. The only decent argument against this method is a shorted cell which subsequently takes out the battery pack and eventually the back-up battery pack. But this isn't what happens, the cell simply becomes an OPEN conductor and passes energy as if it was't even there.

I have numerous GS Aerobatic models rigged this way, in fact I believe that more than 90% of the GS models out there use the dual battery/switch system for redundancy.