Spare tire cover?

Just curious to see if anyone has ever used a spare tire cover on your spare mounted under the bed? Wondering if it would help protect wheel (especially a matching wheel to the other four) or would it cause the opposite by trapping more road grime/dust etc. than without a cover.

 

They trap moisture, which is pretty much the same reason I never use a tire cover on a gate mounted spare.

 

When I sold my set of 5 T4R Pro wheels, the spare that rode under the truck for 3 years was easily in the best condition of the 5. I installed mine face up, to protect the valve stem, but other than that, there isn't much abuse under the bed.

 

If I were concerned about the accumulation of debris I would make a cover out of a fine (and tough) mesh material. This would allow for air circulation to evaporate moisture and protect from debris. Fine dust would likely still accumulate, but I wouldn't too concerned as long as the dust didn't retain moisture. The spare would have to be removed when applying a rust inhibiter to avoid sealing the holes in the mesh material.
(It's engineer in me - I love to explore options to improve things.)