NASA's Mars rovers Spirit and Opportunity accomplished extraordinary feats in science and robotic exploration. Both missions together—including their multi-year extensions—cost around a billion dollars, or about the cost of a single Space Shuttle mission. The rovers cannot replace human spaceflight, but they were an extremely wise investment for what they did.
The problem with the rovers is that there is a communication delay between Earth and Mars. Instructions are sent from Earth and then travel at light speed to the Red Planet. After the rovers execute commands, feedback information must again travel the distance to Earth. Depending on the positions of the planets, this round-trip communication time ranges from 6 to 40 minutes. Imagine having to wait 40 minutes just to know whether your rover got stuck on a pebble or rolled over it. If there were a way to reduce the communication delay, the same sort of rovers could accomplish far more science and exploration each Martian day.
Astronauts in Mars orbit would experience delays of less than a second. A small number of communications satellites could triangulate their signals to reach any spot on the Martian surface. With this capability, dozens of rovers could efficiently explore sites all over the planet. The astronauts in orbit could operate whichever rovers had the most interesting, risky, or challenging tasks, while operators on Earth could manage the rest of the rovers in the traditional way. Sustaining humans in Mars orbit would be a grand undertaking, so even a tremendous amount of science would scarcely justify the effort. But it could justify part of it. It seems reasonable to send humans to orbit Mars as a precursor to a landing, and the ability to operate robotic rovers in near real-time would be one of the many rewards of this intermediate step.