Russ, you're right about sweat, but I think you're off on the amount. The sweat builds at a slow pace, the saddle absorbs some but I've never seen a leather saddle saturated from just sweat, that's just me though.
I'm afraid Russ is right. I killed a Brooks one time on a century ride. Miserably humid, my shorts were dripping from sweat 30 miles into the ride and then it got hot. Proofide didn't keep the sweat from soaking the saddle through by the time I sagged in the last 15 miles. Since then there's a saddle cover in my bar bag on any ride over 25 miles.