The only derailleur cable that I have ever broken (in over half a million miles of riding) broke near the front derailleur. Fortunately, we were near the top of Petrified Forest Rd, about to drop into Calistoga in Napa County so we could just lock down in the middle ring and roll into town for a breakfast break while we waited for the bike shop to open. While the shop didn't have any tandem cables, my tandem doesn't need those since the stoker handles the shifting.

I'd only carry spare cables if I was going to be very far from a shop with a lot of interesting terrain to deal with.