I think you're forgetting the reason the wood is expanding is because IT IS ABSORBING WATER. When that water evaporates yeah it shrinks back down... but in the time it took to evaporate from the surface, nothing was evaporating from the center. If the wood sits in the rain for two days then takes 5 days to dry out to the same dryness it was before the rain you just lost a week of drying time. Without the top being covered this happens over and over all year long.
Only on the internet will you find people convinced that getting something wet will cause it to dry out faster. :bang:
and btw if it takes your wood 5 days to dry out after a rain you may want to reconsider how and where you pile it