I've put together my responses to the arguments I've seen in defense of the programme.
It will be damaging to scientific research.
No. There has never been a strong science case for humans to perform science research in space, other than field geology, which robotic apparati still can't do well. But for most in situ scientific experiments - spectroscopy and particle measurements, for example - a human will have to use a machine to perform the analysis. Such machines can be operated well from remote locations, e.g. the Earth.
In fact, since the proposed budget will boost robotic spaceflight efforts, this change is likely to be beneficial to space-related science research.
It will be damaging to public perception of science.
This is probably true for as long as the story remains in the popular press. I would argue, though, that the mysterious entity commonly referred to as "the public" seem to have become rather blasé about the most spectacular aspects of human spaceflight associated with science, such as shuttle launches and the development of the ISS, in recent years. Scientists are still a fairly stereotyped bunch. The press they get still presents them as Martian boffins slaving away in dark rooms and allowed out quarterly to show their results to a public which suspects them of probably getting it wrong anyway.
Seriously, though, I would argue that this temporary damage is founded on a misconception of the purpose of science. I think technological development is often confused with scientific research. They are sometimes linked, but the distinction is important. The jobs that will be lost due to the cancellation of Constellation, because they support human spaceflight, will mostly be technology-based. Technological developments that ultimately improve science experiments do not have to depend on their application to science. Likewise, basic research in science usually has tenuous applicability to technology. Figuring out whether or not it's useful is frankly not the scientists' job.
In the long term, however, a positive public perception of science will have to be earned in arenas much closer to home. The one that springs instantly to mind is climate change.
It will allow other countries (e.g. China) to return people to space before America does.
This seems to me to be the most tenuous defense of the human spaceflight programme, and also the most challenging to counter, because the motivation is political, not scientific or technological, and therefore not necessarily logical.
Let's put aside the fact that I don't see this as the calamity that a lot of other Americans do. I think that it's far more likely that a commercial enterprise will establish a regular spaceflight service (whatever the destination) before a national entity does. They have the financial resources and the willingness to take risks that governments do not. In the worst case scenario, astronauts dying in the service of corporations is likely to cause less outrage than those who die in the service of a government, because public money wasn't used to fund the failure of their mission.
There isn't much point in trying to cow the Russians with the might of an American spaceflight programme since they are now almost inextricably interdependent. The entities with which America is actually engaged in war at present are hardly likely to be demoralized because America wins the race to establish a moon base. And I'm sure there are more efficient and inexpensive ways to counter the Chinese threat to America's position as global superpower than by beating them to building a moon base.
I have more immediate concerns about the UK's funding of space exploration to worry about. The 2010-2015 plan released by STFC (Science & Technology Funding Council) proposed a "managed withdrawal" from both of the robotic space missions I work on - Cluster (at Earth) and Cassini (at Saturn). These missions are still operational and NASA's support for Cassini, which is a joint mission with Europe, is expected to continue for another seven years. Withdrawing support for instruments built and operated by UK institutions for an international flagship mission seems to me to be the height of foolishness.
There is little I can do about it, however, since I know that our instrument team has been guaranteed full support only through September of this year. I have to start thinking about finding another line of work. Much as I don't want to leave my job, because I love it, I have to remember first and foremost that I am an American who chooses to work in the UK. Retaining my points-based visa depends on maintaining a certain level of income, which means that I can't afford to be sentimental or overly choosy. If I can't find another job doing post-launch support for robotic spacecraft - and there is hardly a glut of such specialist positions going spare - I will have to change fields. This is something of a blow after having just done so. It's been over three years, but when you spent the preceding 12 dedicated to a different one, that doesn't feel like very long.
In conclusion, have a picture of a cat washing his toes.