If we can't manage to land on and live on the nearest planet that has water and other resources given the current technology, just how are we going to mine the asteroids? There is the whole no habitat in space, no way to maintain bone density in long term space missions, no way to land then actually mine the asteroids at the present time. No way to return the materials, just what are you going to do for fuel once you get there? If we can't manage to land on an entire planet how are we going to land on an asteroid that has a non stable orbit not to mention things like tumble?
The asteroid mining is a great concept but it's beyond the steps we would need to do to simply land and survive on Mars. Hell if the Mars atmosphere is not sufficient to shield the astronauts from solar wind, radiation and other "bad" things, how will an airless rock do so?