[eholk]

I see it as a way of focusing the discussion. The numbers we could plug into it are mostly all wild guesses, but the exercise of plugging in different numbers can still be instructive.

For example, we now know there are a huge number of planets out there, and even planets in the habitable zone. Yet we haven't found any extraterrestrial life, so that suggests maybe the other factors are smaller than we initially would have guessed.