That's the reason that caused Bill Boston to develop this design back in the 1970s, but then he was working with 14T small freewheel cogs. Today you can get cassette cogs down to 9T and Terry just continues the old design as a marketing affectation. Apparently Terry customers don't mind carrying two different sized tubes, and on tour, tires.
tcs
I always carry a couple of tubes with me when I ride. Having 2 different sizes of tubes is not a big deal. And if you are going to do the riding that requires carrying tires with you, these days the folding ones are not that big and with that kind of riding you will be carrying extra stuff anyways, so what is an extra tire or 2?
I think that the design is kind of goofy, but I never had the need for one, on the other hand they are people who are loyal customers. Whatever works for somebody...