Yes Terpe - I (eventually) saw that definition hidden away under the UKVI's Surinder Singh case link. My complaint is that the link on their eligibility page for an EEA citizen clearly includes the UK: "You can apply for a residence card if you’re living in the UK with a family member who’s a European Economic Area (EEA) citizen." Pretty sure that would be a case of misrepresentation in the commercial world.
As to why, well my wife is here on a UK spouse visa and it seemed like the residence card was an easier option for us than travelling to Edinburgh to obtain a schengen visa for Spain.
Peter.