So, why astronauts?
Consider: Apollo used a noticeable fraction of the U.S. GDP to put six pairs of men on the moon for a total of a couple of man-weeks. For three to four orders of magnitude less money, the Spirit and Opportunity Mars rovers have gone much farther, and seen more, and are still working away more than a year after they landed. Apollo returned samples, which we have not yet done from Mars (but have from deep space). The thing is, sample return does not require people.
The choice is not robots versus people, it's robots versus robots-and-people. Consider just the fingers of a glove of a space suit suitable for use on the moon. The stiffness from the pressure difference is so high that the hands of serious athletes are cramping and exhausted within hours. Next generation space suits may use mechanical hands outside the suit, operated from within. Such waldoes are already standard practice in many kinds of surgery, deep sea research subs, and some hazardous materials operations. Notice that the control for that waldo can go in the space suit, or in a vehicle, or in a spaceship, or in an office in downtown Houston. One of those doesn't require a mission-critical life-support system.
In fact, there is no choice. There will be robots now, and there will be robots and people later. In between, we can launch people if we like, but they won't do much.
The problem with manned spaceflight right now is that the next step is really hard. It will be a long time before we are able to do something obviously new and exciting. But for unmanned probes, there are dozens, perhaps hundreds of interesting places to go, all within reach of rockets we have today.
Our destiny will arrive on its own schedule. People will work in space when it makes economic sense to do so. Eventually those workers will have significant needs and desires of their own, best satisfied in space, and... off they go.
When the U.S. and the Soviet Union began competing for world mindshare by pulling ever-larger stunts with rockets, NASA got the job of putting men on the moon (and returning them safely).
The prestige mission of men in space can be retired, just as the B-1 bomber fleet was retooled to carry conventional weapons instead of nuclear ones. It was useful, and it is over. NASA should get back to doing basic space science. Telescopes, nuclear power and propulsion, extraterrestrial geology, that sort of thing. They will eventually find the killer app up there. They have not yet, but there is no need to be impatient. Meantime, we should enjoy the pretty pictures and exciting discoveries.
P.S. If Michael Griffin ever reads this blog, can you please point out to your mission designers that the most important instrument on every probe is the high-resolution camera? The images returned are the data best interpreted by the people who are paying for all this science -- the U.S. public. The other science is important too but none of it matters if it doesn't matter to Joe Sixpack. Mission designers who do not agree should feel free to get private funding of their probes.