One of the most common conceits in science fiction is that the galaxy is filled with habitable, Earthlike worlds. Though some attempt to justify it one way or another - Star Trek, for example, posits that some staggeringly ancient alien species specifically seeded Earth-type life across the stars - the fact is that this strikes me as a thinly disguised crutch. While we don't yet have enough information to conclude whether or not the Rare Earth hypothesis is accurate, I'd think an accurate depiction of humanity amongst the stars would include far more hostile planets than welcoming ones.
Another conceit that feels like it's started gaining more ground recently is that because of the above arithmetic, depictions of humans living on planets other than Earth is fantasy. On January 4, someone with a Google account who I can't identify commented along those lines to Randy McDonald's forum question about manned space travel - that "Earth is our only credible home." Charles Stross went one better when he wrote his The High Frontier, Redux a while ago, which when I read it galvanized me to never, ever buy anything Charles Stross writes. Yes, of course, it's absolutely ridiculous to consider the dangers of having humanity all on one planet, where it could be wiped out by an errant asteroid or cosmic disaster, because since "the future extinction of the human species cannot affect you if you are already dead: strictly speaking, it should be of no personal concern." Because interest in the well-being of future generations is for NERDS, right? Burn all the dirty coal now! But I digress...
Personally, the idea of Earth being humanity's only credible home never rings totally true to me. With present technology I think it would be possible, albeit vastly, vastly expensive, to support populations on Luna or Mars. The biggest credibility gap isn't technological, but economic, and with hope that's a gap that will narrow in the years ahead - presuming that our nineteenth century infrastructure doesn't go to pot in the meantime, or that future recessions don't act like a boot stamping on a picture of a human face - seemingly forever.
It doesn't ring true to me because not even Earth as a whole is a completely "credible" home for humanity. I can't help but wonder if a lot of the people who follow the "humanity's home is Earth, and that is that" ideal live in Southern California, England, or other salubrious areas that have generally stable climate patterns. The climate of Los Angeles, say, is remarkably stable and temperate throughout the year - except, of course, on the occasions when I am there - and generally never too hot or cold. The fact that I was able to walk along Venice Beach in a T-shirt in December is proof enough of that.
I, however, live in Canada - a land that is manifestly not credible, in my opinion, as a home for humanity in respect of humanity's natural capabilities. As I write this the temperature in Toronto is hovering at 0° Celsius, and it is unusually warm today. Ordinary Toronto winters see multiple days where the cold is such that it is dangerous to go outside. The only way humans can cope with this climate is through technology - from coats of animal fur on one end of the spectrum, to central heating on the other. Drop a naked person out on Yonge Street on an extreme cold weather alert day, and assuming no intervention I don't think it would take that long for them to freeze to death.
Living on hostile worlds isn't some foreign experience to humanity. Hostility comes in degrees, and so does our capability to cope with it. To claim that Earth, or our association with it, is somehow "special" does a disservice. Humanity is suited for Earth, sure, but it's not suited for all of Earth - and besides, most people tend to have more than one suit hanging in their closet.