Steve Squyres presented at the Augustine Commission and said EXACTLY what I had been thinking on the matter. (see their website for a PDF of his words)

Send humans out there - if they significantly improve the scientific return.

We've not even scraped the surface of what robotics could do on the moon. Where are the stereo HDTV rovers? Where is the robotic sample return by ESA and the US? Russia roved the moon in the 70's. The US hasn't even tried yet. You can drive, in real time, with scientists right there with you, watching, controlling cameas, in real time. Zooming in on interesting rocks, in stereo HD, genuine telepresence could bridge that gap that Apollo 17 seemed to bridge by taking a geologist to the moon.

I've not seen a scientific case made justifying a return of astronauts to the moon. I'm not saying a case could not be made, but we've not even tried to push the limits of robotics there yet. The GLXP I think will result in some valient efforts, but the budget is not enough to push robotics to the limit on the lunar surface. think MSL sized vehicles roving for months or years, covering dozens and dozens, even hundreds of KM's.

Whereas - Robotics can only go so far on Mars. The one way light time means you will always be stuck, with a rover that can only do what it's been told, and onboard intelligence, for a day. Spirit and Opportunity have done bloody well. MSL will do well also. But we could do SO much better on the moon.

It seems GWB said "Lets put people on the Moon" without first asking why.