The answer depends on the objectives of the mission. Geologists, geophysicists, biologists, and others all have different questions about the red planet. The current solution is to build site-specific rovers. For example, as Sojourner showed, planning a route through a boulder field is slow and complex. The time it takes to send and receive messages between Earth and Mars introduces further complications.
People can adapt to changing situations.
Astronauts can pick an alternative route, or even redefine a mission's purpose, in a much shorter time than it takes engineers on Earth to send commands to a robotic rover. They're also better at responding to problems and working out solutions. For instance, an astronaut would recognize an impact crater and know that it is to be avoided. Humans are also very good at performing lots of complex tasks, thereby optimizing the necessarily short time they would have on Mars's surface. Robots can help greatly in our exploration, but there is still a very real need for human exploration.