Robots don't get anywhere near the effectiveness of boots on the ground, even with the most advanced robotic explorers we have today. All of the science done by the Martian rovers could have been done by a human geologist in a few hours work (except the traversal itself, so let's add a day's hike on top of a few hours lab work). And the robots are only really able to inspect the stuff that happens to be exposed on the ground around them and easily reached by limited robotic manipulators.

Sending humans to Mars for even a 30-day mission would be orders of magnitude more science return than the entire robotic exploration program to date.

And that's without considering relativity time-lag. There's serious consideration of an orbital precursor mission to Mars that would put robotic handlers in Martian orbit, or a free-return trajectory near Mars, without a lander, to do little more than remote control a couple of Curiosity-style rovers that accompany them (and maybe visit one or both low-gravity martian moons). It's anticipated that the time-lag reduction alone could vastly increase the amount of science that could be done on a robotic mission before the nuclear batteries die or servos get clogged with dust.

And of course all the robotic arguments are predicated on basic science being the only reason to go in the first place. In reality, most of the people interested in Mars and space exploration generally want people to permanently settle in space. I actually don't give a damn whether there was once life on Mars.. but there damn well better be human societies there within my lifetime.