Carrying a spare cable/ doing a repair like that on the road seems crazy to me. I can't imagine having the patience/ tools to do it well enough, and a substandard repair is just setting yourself up for repeat failures. If a cable ever breaks on you, you can pick a gear and clamp it under a bottle cage bolt. You'll still have the front shifting (or rear if the front goes) which is enough range to get home. Carrying a spare cable is basically invoking the wrath of the cycling gods.