Every species that humanity pushes over the line into extinction is a lost opportunity to learn; billions of years of evolutionary process went into optimising the behaviour and form of that creature, the only way we can learn about that particular tendril of the evolutionary tree following extinction is to study remnants, echoes, fossils. It is important for the survival and optimal development of our species to only let this happen if absolutely necessary – but not to prevent it at all costs.
As far as we know, Homo sapien sapien are the only species on earth who have ever reached a point of such complexity in the evolutionary process where we could intentionally and realistically, start to propagate the life that has developed on our planet to-date, across space and onto other worlds. We could do that today. We may have done it already, unwittingly – there could be colonies of terrestrial bacteria developing and evolving on Mars, Titan, perhaps even the Earth’s moon, as I write.
Life on Earth is constantly – and has always been – running a gauntlet. Every few million years, an asteroid or comet strikes our planet – a direct hit. When that happens, most complex, highly evolved life is wiped out. The process is reset. It will happen again, maybe tomorrow, there is no “if” – just “when”. We have no reliable way to tell “when” and even if we did, currently it probably would not make a hell of a lot of difference – depending on the force of the impact, our species (along with many others) could be wiped out, regardless.
[Vredefort impact crater, SA (2 billion years old) - http://en.wikipedia.org/wiki/Vredefort_crater]
As I see it, it is naive to say “save the planet”. The planet has seen far, far worse than Homo sapien sapiens; she has the scars to prove it. Don’t worry about that. What we need to think about is how we can optimise the rate of development of our species’ technology in a way that will enable us to preserve as much of the precious and fragile life that has developed on Earth as we can – the ultimate objective being extra-terrestrial propagation and thus, redundancy.
[edit: 2013-05-01]: Clarification on this matter is here - http://blog.infostructure.co.nz/2013/04/clarification-planet-does-not-need-to.html