There's a general principle for in-situ resource utilization that you want to eat the whole pig from the nose to the fingernails and twisty little tail.
Also depends what it is. In many situations (la Luna) volatiles are precious. I'm not so sure you want to harvest water from the Moon's north pole and use it to make fuel that will be consumed as opposed to maintaining an inventory for use in a circular economy.
I'm not sure what attitude about volatiles Martians would have. It might be able to support an atmosphere if the planet could be protected from solar radiation.
