But BEagle, you stopped before the good bit!
'NOTE: For aircraft of 5700 kg MTWA or less, an acceptable means of compliance is to placard and restrict non-immune receivers to operations permitted under the Restricted Approval Category LA Class 3 (see paragraph 5.4), irrespective of the approval category for that equipment.'
LA Class 3 equipment may be used for the 'facilitation' of any flight (IFR or VFR) outside controlled airspace.
Nevertheless, your point stands that anyone contemplating an aircraft for serious IFR would be nuts to pick one without FM immune equipment.