I agree with Daniel: going to space is about exploring new places, about discoveries, not about putting a segment of Earth's population on another planet.

But exploration alone cannot justify the money spent on it.

There are always multiple reasons for space exploration. Those promoted are not even those really behind the minds of those who have the money.
We go to space for science , but it's about being the first one in this field.
It's about arriving first on the Moon.
It's about rivalry, different conceptions of governments.
I'm not impressed either when the terraforming issue is raised. It would take far too long to justify colonizing a planet.
Our planet being engulfed by our sun an therefore we have to leave the solar system before it's too late (that's a favorite theme in conferences and for children) but it forgets our Earth won't be able to sustain liquid water much earlier than when the Sun will collapse. It forgets every animal species starts, thrives and eventually extinguishes, so will the human species. It forgets our cultural development has exceeded our biological evolution.

We go to space and somehow we keep sending humans even though we can't shrink them or they produce very little science as robots can be built for a specific research and be excellent at it.

If there are humans in space, at least in the US point of view, there is I believe a "not openly admitted reason" for it:
Manifest Destiny.
It's politically unthinkable to abandon space even though it's not practical to have humans in space. But a manifest destiny looks far too much like imperalism. However, space is the ideal place for manifest destiny to express itself.
This could however mean that a five year long hiatus (between Shuttle and CEV) could be acceptable, as long as there is the promise for the next vehicles and it could also mean the CEV could be mainly used in low Earth orbit rather than towards celestial bodies.
Anyway, countries will get the space progrzam they deserve.

Raoul