With a cover, you're less inclined to check the tire pressure of the spare. If you get a flat, you run the risk of calling an under inflated tire into service.
And I love the look of my wheels, so for that reason alone I leave it bare. If I did cover it, I would want to install a custom-designed tire cover.

Chris here. I drive a 2-door Velocity Blue 2023 Ford Bronco Badlands with Sasquatch, Lux, a 2.7L V6, and an automatic transmission. It’s fun.
"Stupidity is a more dangerous enemy of the good than malice." - Dietrich Bonhoeffer