I really don't get all this talk about worry.

First... It isn't like it is a part that fails often. If you check your cables once in a while you will probably never break one. I have been riding deraileur equipped bikes since they became popular in the US and have worn out a few cables, but have never had one break.

Second... Even if one breaks you can rig that derailleur to be in the gear you choose and still shift the other one. So it isn't like you would be stranded.

Carry one or don't. The risk is small for not carrying it and the penalty is small for carrying it. Personally I choose not to, but either way it isn't something worth worrying much over.