Ever since Rare Earth, there has been a consistent attempt to show that habitable zones not only exist, but that that
1) The emergence of life so fast after Earth became habitable is not extremely rare (for if, as Ward & Brownlee argued and has been argued perhaps most recently in a 2012 PNAS study, abiogenesis on Earth is a typical, and would require significantly greater time periods, than the number of habitable zones becomes far, far less important)
2) That the complexity of life on earth (and by this I mean multicellular animal life, not little green men) and the dynamics of the evolutionary processes here are again not atypical. basically, that the fact that microbial life is far more adaptive to just about any environment, Earth's biodiversity can be considered probable or likely granted that life has emerged.
3) That the descriptions of habitable zones are not overly optimistic. We've had, if memory serves, 5 mass extinctions on this planet, one which wiped out most life. A recent study in the journal Astrobiology looked at solar-driven climate change and determined that the relative stability that allowed Earth's biodiversity is very much atypical, which others are trying to show can be considered differently (andother study in the same journal discussed the fact that even though cosmic rays and ionization are harmful, and that our solar shield is unique, it there are ways in which these negative factors are or could be positive).
Not so long ago (before the advent of complex systems research along with other changes in our understanding of living systems), it was sort of assumed that all we needed was a habitable planet, and even if they are extremely rare this doesn't matter because there are so many. That's changed. The astrophysicists who argue that multicellular life is extremely rare and perhaps even unique to Earth have identified and increasing number of factors which makes the probability of complex life elsewhere vastly more improbable. The responses to such arguments often just involve playing around with variables.
Humans are biased in a particlar way, which makes us naturally bad when it comes to probability. We know life evolved here. We know there are an enormous number of factors involved in life evolving, and we know there are an enormous number of planets. But as we don't know all that much either about life or about the number of "habitable" planets, you can pretty much pick whatever variables you want and plug them into a model, which can demonstrate that complex life almost certainly exists throughout the universe, or almost certainly doesn't.
