Why own extra tires? Tires are expensive, unlikely to fail unexpectedly, and can be purchased quickly. I'm looking forward to hearing why others think it's important to keep all these tires around for so long that rubber degradation is an issue.
Yes tires and tubes can be outrageously expensive if you buy them on the spot from your local bike shop. As you do. Little more reasonable price if you buy them on sale over the internet. I don't like wasting money on tires and tubes so I buy them when on sale. And buy them in bulk since I have lots of bikes. Tires and tubes can and do fail unexpectedly. Usually tires wear out over time. But they can just give way all of a sudden. Its good to have spares at the ready. Not at the bike shop where it takes time and effort to get to. Rubber degradation? Never given it much thought. I think that occurs over decades, not the few years I keep spare tires and tubes around. Not something I spend much time or effort worrying about.