I'm surprised you haven't seen it. I've only been diving for five years and I've seen a fair number of tanks with rusty spots where the boot used to be. Maybe it's a function of you not seeing the singles without the boot - you don't see what's under the boot. The rusty (or whatever you call the white corrosion on al80 tanks) tanks I've seen have all been sitting in dive shops waiting to recieve (or be picked up from) vip or hydro. It's just a common enough problem that I've seen it numerous times in passing. I can't say whether or not the corrosion is anything other than a cosmetic problem. Maybe it just doesn't matter.In my part of the world, I hardly ever (if at all) see a single tank without a boot. And I hardly ever (if at all) see a twinset with boots.
What's interesting is that most of the twinsets I see have scratched bottoms, often with some rust. I still haven't seen or heard about a single that has had to be scrapped due to rust under the boot, even if I don't doubt that those things do happen. But ISTM that if one is reasonably diligent about rinsing the tank(s) after diving in saltwater, rust under the boot won't be a significant problem.