There is one thing I can think of where the instructions would be to NOT rebalance the tires - and that is if the test is to see if the balance changes over time - like a spray-in sealant was applied to the inside and there is a question of if it will flow over time and cause an imbalance.
That is one of the things I considered. I think these were the prototype tires designed to be built using bias construction machinery (flat belt radial?). I also considered that balancing might throw off something they were tracking. This was in the days of the dreaded 55 mph speed limit, and my memory (which is imprecise at this point) is that the tires were pretty stable up to that speed. I cannot promise you it was never driven past 55 by anyoneās son, but I didnāt drive that car very much.
Weāre down to 5 years now? Anyway, the reason is driving slowly on an old spare is still better than driving on a flat tire or bare rim.