I've never taken the batteries out solely to dry things out. Typically, I book-end my week with bike commuting (I work four days/wk). I ride one day, then ride again on the 4th day. I charge my batteries after the second commute of the week. If they get wet on day 1, then they stay wet/or dry on their own. If they get wet on day 4, then yes, I take them out to charge them, not to dry them.

I have done absolutely nothing to deserve the holders lasting so long (3.5 years so far). If I was to guess, a major factor is probably that the batteries are mounted on top of my rear rack. The rack is protected from spray by a fender. Naturally, the rack is behind me, the rider. They probably don't get very wet most of the time.

I have used them perhaps 15 to 20 times with it mounted by the rear wheel on a bicycle child trailer. In that set up, the battery pack is close to both the ground and the trailer wheel. A few times while doing this, I've been rained on pretty hard, and they probably got soaked.