I love all the stories about the search for water and possibly life on Mars and the new discoveries of more earth-like extrasolar planets, but I just think there's a bit of a perspective problem when it comes to presuming that being closer to earth-like is better when it comes to the search for life. Yes, the only known sample point we have of life is that existing on our own planet (for now), but that doesn't mean that our planet is optimal for the formation or survival of life.
Let's just put this in persepective, as far as we know today it took close to three billion years for life to reach the multi-cellular stage (regardless of whether you're in the panspermia or abiogenesis camp), and today, after four billion years, only 0.00000000126% of the planet has been converted to biomass.
That's a real long time and not much progress to show for it. We may not be living on a fertile world, an optimal environment for life; maybe we live on the galactic equivalent of a barren weed strewn city lot.