Please correct me if I am wrong:
Some gas appliances might have to be lit and re-lit manually if there is an electric failure. E.g. my oven and stove top burners. Other gas appliances might have safety devices which fail closed if power is absent. E.g. my hot water heaters: I can re-light all day long, but unless there is electric power to the shut-off... it shuts off. So... while I consider natural gas an important preparedness addition, I don't think it would do anything for me except run the stove/ovens.
ALSO: I think the burner jet sizes for natural gas differ than for propane. So don't assume your propane bottles will run your natural gas appliances without some modifications.