The notion that rock in a post hole will keep timbers relatively dry is a little sketchy.
Without a barrier (like filter fabric described above) soil will eventually fill the voids between the rock. Granted there will be less water-holding capacity in a rock-soil mixture than a straight soil mixture but the moisture content will basically be the same as the surrounding soil. I doubt water is draining though the rock as it would through a sieve.
Once soil is saturated water moves mechanically to areas of lower moisture content or lower pressure...even into a huge void such as my basement.
