Bringing an asteroid back to Earth? What's that have to do with space exploration? If we were moving outward from there, and an asteroid is a good stopping point, then fine. But now it's turned into a whole planetary defense exercise at the cost of our outward exploration.
Mars could very well be a staging location for the resources of the asteroid belt. We have to learn how to get a payback somewhere, but it's beyond Mars that the real payoff will come from minerals.
Mars is the symbolic and totally stimulating next objective that could so dominate the next century's exploration efforts. From Mars, the resources of all the asteroids will become readily available.