I see the logic behind this, but when I made the decision to have a spare with me for my last trips, I figured that if I was going to spend the money on a folding tire anyway, plus the fact that the chances were good that if on the slim chance that I really damaged a tire, then I might as well just have the same model as on the bike, and then just use the spare and be done with.
It did only weigh 560g, so the diff in weight is there I guess compared to a really light 26 folder, but then I kinda figured that I might not be able to find a good replacement.
I really do think the main factor for anyone to take into account is the place you are going, and likelihood of finding a good quality tire that wouldnt cost an arm and a leg.
Last summer I crossed France and didnt bother taking the spare, figured it really wasnt an issue there for bike stores and tires.
I have not had a tire blow out or otherwise become unrideable since I was a kid, so I agree it is a highly unlikely event. In the past decade, I have had three tires develop bulges where it was clear that the internal fabric was rapidly failing, but I managed to get home and change the tire before a blow out.
If I carry a spare tire, I see it as an insurance policy, I am paying (in weight) for protection against a highly unlikely event. Before I bought that light weight tire, when I carried a spare, it was a replacement tire that was the same as I was riding on. I did not intentionally go out and buy a light weight tire to carry as a spare, but when I saw it on a clearance price, that is when concept of the light weight spare came to mind. Kind of like the tiny little spare tires on some cars.