Manned space exploration: good investment, or not?

Great debate via the excellent Freakonomics blog.

While I'm very firmly an advocate of human space flight (I'd say the lame pursuit of human space exploration is one of the biggest failings in my lifetime), I was stunned by how thin the arguments are for human space flight.

The arguments for human space flight basically summed to:

1) it's a good investment, as development of the space program has generated inventions from velcro to GPS, etc.
2) we should do it because we're explorers, or because we need another planet to balance the fact that the Earth as we know it could vanish thru human activity (war, environmental crises) or external threats (asteroids, etc.)
3) it doesn't cost TOO much (with variations on the theme of "it's a tiny amount when compared to.....")

These aren't exactly overwhelming arguments.

Personally, I think the quest for Helium-3 energy is a compelling argument for human space exploration (or microwave-transmitted solar power). Beyond that, I can't contribute any other compelling argument either, though I KNOW we should be doing more of it.

The counter-arguments are classic: there's a better use for the $$$ curing social ills here on Earth. I can't argue with that, as there's clearly a need, though the libertarian arguments of "how about we just not have the government spend the money at all," and "if space exploration is so worthwhile, why not let the private sector do it?" have resonance with me.

These, and other arguments are coherently put forth in reader comments, which is something that I also found interesting - I got as much out of my reading of the reader comments as I did out of the editorial. In fact, the reader comments collectively were a stronger counter argument.