I was about to contribute something along the lines of contributory power systems, as opposed to primary power systems. Chu Chu makes an interesting point.
Consider the situation with cruise ships. Several designs have now been created that have big automated sails as well as fossil fuel power sources. These significantly reduce the fuel requirements for the journey, and aren't just for pose value. Cost/value remains marginal, or they would all use them.
Consider the aviation world again. Personally, I rather like the idea that the A320 or 777 with 2 engine failure and a loss of electrical power might be able to rely on alternate PV power for the instruments/radios/computers/actuators enabling a controlled glide descent to an airfield. I don't imagine that you'd need too many m2 of pv to power the electronics.