2016 2.5i Premium / 2020 2.5i Premium / 2021 Rav4 Prime / 2012 F150 ecoboost
Yeah, you are probably right, because of the AWD.I think this is exactly right. The whole point of rotating tires is to keep tread wear even across all four tires. If you rotate your tires at every 6k, and you have tires with a 60k expected tread life, that would mean that you have used approximately 10% of your usable tread between rotations. Probably about 20% for the OEM tires. Throwing the spare into the rotation means you would never have even tread wear across your four tires, which is the exact opposite of the whole concept. Keep your spare, either donut or full size, as a spare!
If you'd want to bring the spare into the rotation, you'd have to do rotations much more often, and then it probably would not be worth the 20% or so overall savings.
In a Subaru, I would just keep the spare as the spare.