I used to leave them in occasionally, but not consistently, and I stopped doing that long ago. I think shortly after I noticed the first signs of rust, and before the parts had rusted badly enough to require replacement. They've certainly never been left in the unit for the past year, during which time all of the new replacement parts have started to rust again.
I called Tascam's service department last night and managed to get their service manager (Jim) on the phone. He was very nice and seemed genuinely curious about what could possibly be happening, but of course always directed the conversation back to what I could possibly be doing to cause it. He indicated that, in southern California, they see a lot of these with the XLRs rusting due to the proximity to the ocean, but he was surprised to hear that EVERYTHING was rusting. I described the observation with the metal snaps on the carrying case rusting only where in contact with the metal on the unit, and he acknowledged that it was strange. When I brought up the galvanic corrosion theory, he seemed to think that not all of the required elements were in place for that to occur ("there's not a complete circuit"). Although he admitted that he's not a chemist nor a physicist and was only recalling what he had learned in high school. Which was quite a while ago, since he said he's been in Tascam's service department for 20 years. Also, he thought the rusted screws on the bottom of the unit couldn't be related to this because they're supposedly not in contact with any metal (just plastic). In the end, he took my name and number and said he was going to talk to the engineers and maybe even "call Japan" to see if they have ever observed this. Since he's going on vacation next week, he said he'd try to get back to me within a couple of days. There was no mention of offering to replace the unit if it's determined that there might have been a problem with it from the start. But early in the conversation, when I mentioned having already paid $250 to replace all of the inputs, he said they'd be happy to do the replacements in the future for a much better price. Ummm....no thanks. That was early in the conversation, though, and by the end I think he was feeling a little sorry for the situation.