I carry two spares! There was a time when you could count on a buddy to provide an extra spare if necessary (I once had four flats on a rainy sixty mile club ride, and was having trouble keeping track of all the people to whom I owed tires) but it's a rare bike ride today when I'm not the only one using tubulars. So it's a very serious concern to the modern day rider, and one of several very good reasons not to use them.

I put them on a "luxury" British-style fixed wheel touring bike that probably won't ever see more than 200 kilometers in a day, so worst-case scenario has me on a bus for a 60 mile trip home. The "serious" LD bike still runs the wonderful and very reliable Grand Bois Hetre 650b clinchers. I'm sure these have nylon casings, but I guess that doesn't matter so much when the tire is 42mm wide!