In the grand scheme of things the 3-4 ounces that a spare cable weighs as well as the room it takes up is minuscule. If it make you feel better carry one, or not. I cleaned out an old seat bag of mine that I used for road riding a long time ago. There were a spare brake and a spare derailleur cable in there. Probably been in the bag over 20 years. Obviously never needed on a ride. Funny thing though, on the way back from the grocery store the other evening I broke the shifter cable on my city bike, Nexus 8 hub, no spare on the bike or in my shop. Used my last spare on my son's bike and forgot to order more. Oh well, trip to town will take care of it.

Aaron