Let's hit a mountain!
The reason for the FM immunity regulations was that the aviation authoritys in europe had let commercial radio transmitters to broadcast right up to the bottom end of the aviation band.
This may effect the performance of aircraft navigation kit in the VHF range.
It would seem to me that all this talk of the leagality of using non FM immune kit in IFR is just stupid, after all the mountain or tall building that you hit is unlikely to know if it is in class A or G airspace.