I see your point Richard. I suppose it would depend if I owned an aircraft or not. If I owned my own aircraft I think I would rather do my BFR in that aircraft since it would be the one I was flying most often. It would give me a vaild test of my skills in that aircraft and perhaps show me what I need to work on over the next year.
However if I did want to enhance my stick and rudder skills, and brush up all around, expand my knowledge and try something new, I think I would just choose a plane to get a good thorough check out with and try my hand at that aircraft....will that be a valid test of your overall skill. Perhaps not since you don't have time in that aircraft but it would expand your knowledge.
I suppose it all depends on what you want to accomplish, if you want to kill 2 birds with 1 stone get 10hrs in something you haven't flown and consider that a BFR.
A BFR is really only as good as one challenges themselves to be. Some instructors have weak BFRs and others are tooo stringent. Should it be taylored to the clients background....perhaps? Above all I think it should be fun, and should be valid...and it should keep the client coming back for their check-up that they often dread.
As a side note...I hated giving BFRs...never knew what you were going to get. Astronauts, Cowboys, or the Ace of the Base....you never knew.