Flying Fowl...
A reasonable argument, under normal circumstances - although you may have said it rather vociferously.
Actually, I did have access to a C172 that was (indeed, still is) cleared for spinning.
During all my conversations with the flight school (which specialises in JAA FIC instruction) I stated my intention to bring "my own aircraft" (not actually MINE - but a recognised turn of phrase).
The school eventually went to great lengths to point out that the transit times for my journey were not worthwhile and that I should use their own PA28 - which they always used for training and testing. As I'd done spinning before in a PA28 - and this was a recognised school - I didn't think to ask if they actually knew what they were talking about.
A previous attempt to renew the rating - one month earlier, with a different examiner, at a different school, did involve a conversation in which the examiner made it clear that the flight would be in a C172...for both W & B purposes and spinning purposes.
The earlier flight test was eventually cancelled due to both the examiner and I being 4,000 miles apart at the relevant weekend.
I would ask you, FF, to note that my post was not geared to attack the examiner (from whom I learned a great deal, as I always do on these flights), but was meant as a "poll" of which was the more suitable option - the overweight, out of balance aircraft that was cleared for spinning - or the non spin approved aircraft that was within its performance and control envelope.
....and I would still appreciate the book references for further study!!