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This is general practice for any spare that is full size and has a matching rim. However it is particularly important on a subaru (or any AWD). In the event that you need to use the spare it will have been worn down to the same diameter as the other 4 tires, this prevents damage to the AWD system.
If the rubber is pliable and free of cracks/splits and the tread is that of new, then it will behave as new. You have yet to provide any evidence as to the quantifiable dangers of an aged, but not worn, tire.
thermo-oxidative degradation. Air/Oxygen and heat can and will degrade the internal layers over time.
The tire may look fine from the outside. May feel find driving on it, but put excessive forces on it such as swerving or heavy emergency braking could stress it beyond its aged capabilities resulting in a blow out or tread separation.
I have 1999 Buick Park Ave with 15,000 original mikles on it and the same tires the day it was sold and they still look and ride like new. The whole car looks and drives better than my later vehicles with higher mileage. My wife drives only to the store and doctors maybe 2-3 times a week. That probably is not the norm for most of you.
The tire dealer I regularly see for air etc (Discount Tire Co) say that the tires are to be changed at the 10 year mark regardless of the mileage. .
thermo-oxidative degradation. Air/Oxygen and heat can and will degrade the internal layers over time.
This is absolutely true, however the outer layers degrade at a faster rate than the inner layers as the oxygen diffusion is buffered. A tire with rubber that is not degraded is indicative of internal layers that are also not degraded. Degraded (oxidized) rubber has very different visual and physical characteristics that new rubber has. Typically dry (loss of sheen), brittle, flaky, cracks/splits are all indicators that the rubber is no good and that internal layers have also experience oxidation. A new tire sitting on a shelf in a warehouse is not exposed to conditions that accelerate material degradation, they sit there, relatively unaffected… it would take decades to experience the same level of degradation as one under normal or test lab conditions. Also, for in-use tires, the conditions of which each set of tires experiences from one vehicle to the next, one climate to the next, is wildly variable. You can't just slap a universal expiration date on tires.
The couple of vehicles that are mentioned that have been garaged and not driven much - it depends on if they have been in "dead" storage, or driven occasionally. After 7 years of dead storage, or even 4 years of it, I might very well think about changing them out. If the car has been driven occasionally, and if you don't intend to drive it far and fast, you might consider wearing the tires out regardless.
It also depends heavily on the driving environment. For me this is blue roads, not much traffic, occasional jaunt into town on 35 MPH posted roads, usually not that heavy traffic. So, for me, a tire failure is unlikely to be worse than an inconvenience. Most of the time I'm driving the Scirocco or Camry 87, both are small light cars, I have had sudden flats in both and didn't have any problems handling a safe stop. That and I don't have any problem changing a flat. A small or frail person would be different.
Contrast SoCal freeways, where sheets of cars cruise in close formation at pretty much 80 +/- 5 MPH. There, yeah, I wouldn't use a tire I had any doubts about. Which rules out not only older, but cheap tires, and I would be particular about how any nail holes were repaired, too.
And, of course bikes are different too. Dodgy tire can get a fellow killed on a bike.
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