GTW
I agree entirely if one already has a good working ADF; however an ADF is about £3000 + installation and for that you could get a really excellent panel mounted (not "IFR") GPS. There's no doubt in my mind which of the two will be better and safer for navigation.
If I had to spend £3000 on a backup for a GPS (GPS being the primary nav device as it should be) I would spend it on an RMI.
It seems a very arrogant attitude (by the vendor) if the original poster is correct. It is very probably legal (amazing as this may sound) to fly an NDB procedure without an ADF if it is in Class G, but that's rather limiting!! Would you pay £400-500k for a plane which right now is definitely illegal in most relevant circumstances in Class D and above?