In addition to that however, sending a human crew to Mars will result in more digging, surveying, water prospecting and life hunting being done within the first month of exploration than all of the rover missions combined.
The added benefit of having humans in space, especially if their reason for being in there is because that particular bit of space is in the way of earth and a planet, is securing the legacy of the human race by putting our eggs in more than one basket.
To that end, I'll flip your assertion back on you: The only reason to send a robot to space is to prove that you can, especially if we don't follow it up with human missions.