I don't know how hot it gets down in the plug wells, but my guess is that it's plenty warm enough to evaporate any moisture in there while the engine is running. The trouble probably starts when you park the bike and there's still water in the plug well, especially if the bike is going to sit for a few days. The cap on the coil stick slows down the evaporation when the engine cools off, and the plug might sit there for a few days submerged in water. I imagine that doesn't have to occur too many times for the plugs to get massively corroded like those I've seen in some photos on this forum, especially if there's road salt mixed in with the water. The trouble with relying on the maintenance schedule when you check the plugs is that, if you wait 4000 miles between checking them, you might still be giving a submerged plug lots of time to get rusty, even though you're diligent in conducting maintenance on schedule. The biggest preventative measure is trying to keep the water out of there in the first place. Even a splash guard, though it could help, may still not totally prevent water from getting on top the cylinder head, and once it's there, it can still work its way down into the plug well. I've had success so far with making a grease seal on the underside of the cap on the coil stick where it makes contact with the plug well, or at least it seems like success since I haven't had the problem occur. The seal may have leaked since I last pulled the plugs though, and my plugs might look like they have barnacles on them by now.
I believe there's a thread on here where a member removed the metal sheaths on the outside of the coil sticks, and that eliminated part of the problem. I suppose the metal sheaths are there to give the coil stick some rigidity so you can push it onto the spark plug, so I don't know if losing the sheath affects that or not.
Alternately, you could just let the engine idle when you get back from a ride when it's been raining. 10 minutes of idling where the engine is running between 205 and 220 degrees might be enough to boil off whatever water might have gotten inside the plug wells.
I didn't have any ACF50 last time I did my plugs. I did wipe the exposed metal parts of them with WD-40. No idea if that'll make any difference or not.