It all comes down to being prepared, there are two end of the rider spectrum, the boy scouts and the weight wienies. Boy Scouts will carry everything they think they might need, tubes, cables, tools, extra clothing, if it's field repairable, they can probably fix it. Weight wienies are those who think that spending $500 more on a new component because it's 5g lighter is a good investment. They refuse to carry anything that might add even 1g of extra weight. This is okay when racing and their is a SAG that can provide tools and parts, or haul your sorry donkey to the end of the line. If not racing and you sometimes ride far from civilization, it can be a really long walk of shame when you break down and find the nearest hamlet is in any direction, is 2 days journey on foot.

Most riders are in between, the more remote area your riding in, the more you need to be toward the boy scout end of the spectrum. On my bike right now there are 3 spare tubes, and 3 spare chain links. I would like to add a couple of spokes and a couple of cables to that on bike spare parts list.