Let's look at his arguments.
Spending on manned space programs means spending here on Earth. This is the counter-artument to the "we should spend money on Earth, not on space." To wit: a space program involves hiring lots of engineers and technicians (most of whom will work here on Earth), and buying lots of raw material (most of which will be sourced here on Earth), and generating economic activity (most of which... you get the idea).
The thing is: robotic programs have the same effect. Engineers, scientists, raw materials...
Technology Spinoffs. Most of the technology spinoffs come from technology-heavy missions, and the robotic missions use more technology to accomplish things other than merely keep the human alive. The example Mr. Plait uses:
But digital cameras owe their existence to Hubble; their light-sensitive chips can trace their lineage straight back to development of the detectors that went on board Hubble's first generation of cameras.Hubble is... a robotic mission.
Achieving the impossible:I have to quote Mr. Plait again, since he says:
But in 1969 NASA looked to this unachievable destination and made it achievable.The flip restatement of this is: because we can. Because it is there.
The thing of it is, if we are going to spend a lot of money on something, it had better have a purpose better than "because we can". Arguments like that get some countries to invade other countries.
National Glory:
In the late 1960s, our culture and our global reputation were crumbling. But for a few shining years we were the envy of the planet. And rightly so. We went to the Moon. NASA's manned and unmanned programs have done incredible things since then, extending our knowledge of the solar system and the Universe to places we couldn't fathom just decades ago...or in other words: New! Shiny! We are so much cooler than everyone else, and doing this will distract the rest of the world from noticing otherwise!
Conclusion
The problem with large, specific-target national efforts, is that once the target has been achieved the public at large loses interest in it. The original Apollo program is exhibit A for that argument.
The goal of space exploration is to increase our knowledge of the universe. When it comes to getting useful-science-per-dollar-spent, robotic missions are better than manned missions in almost every way.
And while manned missions (can) garner more media, national, and international attention, history shows that unless you can keep coming up with visually new and interesting things to do with those missions, the tax-paying public which underwrites these expeditions will question the value of continuing them.
There is also the increased exposure to risk with manned missions. The Apollo 1 fire was bad press. Challenger grounded the shuttle fleet for almost three years; Columbia's breakup grounded the fleet for two and a half.
Now, imagine if the Mars Polar Lander had been a manned mission.
But it wasn't, and Mars robotic programs continue to this day. Since that time the program focuses have been more on reliability than the "fail faster" philosophy that contributed to some of the failures.
There is also the technology argument. New technology costs more, both to design and to operate. Take for example what I believe is the best piece of technology to ever come from the manned space program: the Space Shuttle Main Engine (SSME). This is an engine, capable of lifting payloads to orbit, which is re-usable. It isn't a throwaway. If we are going to run routine manned missions to space, we need a robust technology -- and a reusable engine has to be robust.
Instead, Nasa wants to go back to throw-away rockets because they are cheaper to build and fire. So they want to do expensive programs -- but they want to do them cheaply.
I am not arguing against Nasa's existence. I just think that with the current level of technology, robotic missions are a better investment, both for the returns from those missions, as well as managing the risks facing them.
