Space probes may be cheap but they are slooooowwww. Most of the research done by spirit, opportunity and curiosity could have been done by a trained astronaut in an afternoon. As an example, in its 2623 day mission, spirit traveled less than 5 miles. Rovers are great for low budget exploration, but if you want to kick things into high gear, there's no substitution for trained humans.

Some of you may be thinking, 'Hold on a second, if you took the money for manned exploration and poured that all in to rovers, surely you would get even more science done than humans could possibly do!" Perhaps, but I would argue that the research done to support the manned exploration should be counted in the scientific haul. Meaning that currently, for ten billion dollars, we could shoot rovers to every corner of the solar system using current rocket technology. But if we spent that ten billion dollars sending people to mars, we would have to develop new rockets, new infrastructure, new habitation technologies, cosmic radiation shields and much more. All that technology, plus the knowledge we gain on mars would be more valuable than simply sending rovers all over the solar system, partially because all that new technology would probably make it easier to later send rovers all over the solar system.

In addition to the above, there is also the massive pr power of putting people on mars. Everyone in the world knows Neil Armstrong, but no one cares much about luna 2.