For a long time I believed the talisman theory of cables; carry one and the one on your bike will never break. But even though I've never had it happen to me I've heard stories of it happening to other people.

Seriously, I don't carry derailleur cables any more because it's not a disaster if one breaks. I'd just put the chain on the small ring and middle of the cassette and ride in a 50" gear to the nearest bike shop. Brake cables are a bit different and I still carry one of those as I never want to be on a descent on one set of brakes.......just in case that cable breaks too.