There is also another often overlooked benefit to asteroid mining: the same technologies used to mine an asteroid can also be used to save the world! It is generally accepted that a large asteroid impact wiped out the dinosaurs. If our species is to avoid a similar fate we must have adequate resources in place for early detection of asteroids on an impact orbit with Earth as well as the ability to alter their trajectories.
In searching the skies for mineral wealth, we will chart the orbits of every sizable asteroid that poses a threat to Earth. An asteroid detected early can be moved to a safe trajectory for a relatively small energetic cost. A large mining craft, for instance, could be dispatched to change the course of the threat with its thrusters. Given enough time and a composition of value, the asteroid could even be mined to change its course.