Nobody is going to bring anything back to earth, that would be ridiculous. You'd be better off filtering elements out of salt water.
Mining asteroids is so we can colonize the solar system cheaper.
The only thing that could compete with mining asteroids economically is a space elevator, which would have to be made out of unobtanium and which would attract terror attacks like moths to a flame.
Mining asteroids for gold and platinum
24 April 2012 Last updated at 19:58 ET
The multi-million-dollar plan would use robotic spacecraft to squeeze minerals such as platinum and gold out of the rocks.