When I read Charlie Stross's essay on the impracticality of space colonization, mankind's imperative to survive came to mind as a good counterpoint. As I mulled this over, however, I realized that I was equating life on Earth as we know it now (e.g., suitable atmosphere, large swathes untouched, etc.) with survival; taken more generally, things could get pretty bad on Earth (e.g., mass extinctions, pandemics, a large rise in sea levels globally, etc.) and mankind would still survive, in some form.
That's when I realized that Stross's argument held up: it would likely be more practical to build entire artificial environments on, and possibly orbiting, the Earth than to colonize space (given the technology that's feasible today, as Stross states).
That said, I decided to write about this after reading John Tierney's article in the New York Times on Dr. J. Richard Gott III's theory regarding the survival imperative. Initially, I wondered whether Dr. Gott covered points I hadn't considered. Unfortunately, however, he makes a simplistic leap from our survival to space colonization; if it is based on logic, as opposed to the popular talk on space, it isn't clear in the article. For example, Stross outlined why basing arguments for space colonization on our history of colonization on Earth is flawed.
Also, while the idea that all of our 'eggs', so to speak, are in the single basket called Earth intuitively compels us to colonize space - as I readily admitted above - Gott's seizing on Mars as a solution doesn't stand closer scrutiny: the biggest threats to Earth may also be threats to Mars (e.g., to be very optimistic, the lifespan of the Sun), so we certainly couldn't stop there. Which brings me back to my earlier point: while spreading ourselves out makes sense, for reasons outlined by Stross, things would probably have to get really bad on Earth before space colonization was considered practical.