The distinction I am drawing is between stuff that is useful as a back-up, and stuff that is useful at a completely different time. Where I sweat my decisions is between stuff that might be useful on the trip, and a lighter path where I don't take that stuff. My tower computer is easily upgraded, but I don't pretend it's a netbook. Additionally, for most uses, heavy touring machines are already overweight contraptions designed for every contingency.
In retail there are a lot of things that while they represent some perceived benefit, are mostly there because the person couldn't make up their mind. Chris hinted at this when he mentioned that R. was wiling to implement his vision. But the reason you run into that resistance from frame builders is they run into folks all the time who want a bike that would be good for time trials, though it would be nice if it worked offroad, and could carry a baby. It all seems reasonable to the client.
Well, it is more than a few extra bits. We are talking on this build about two complete parallel set-ups for brakes, and three for gears. Providing access for the gates belt is a non-trivial modification. A stay bridge like that alone, is around 200-300 for S&S. Rohloffs use a lot of cabling, the drop is different, the rear stays are beefed up differently. If you really want to have a nice set-up you need custom bars, at least for drops. Then what you guys are forgetting are the cost of the gates parts. These are expensive! And then what you are also forgetting is since the "second bike" is just "in case", you don't have to even buy it for now. You can invest that money. The average person with these tastes, when the future unfolds is probably going to want a different bike. So on an equal economics basis, two frames, no S&S on both of them, no gates, no financing, and cost the part of the future risk that isn't going to be handled by this. Seems like a wash to me.Even ignoring the S&S couplers, it's much cheaper adding a few extra bits to one custom frame than to have to buy two custom frames.
I'm assuming Chris has made the perfect bike for him, and the execution is fantastic. But I am just not boarding the bells and whistles train myself. I want to make a lean and specific tourer for myself, and my upgrade path is to do it again if I find some changes I want badly enough in the future. Make the hard choices, and if they don't work out make a correction. Belts or suspenders, not both.