There's a great West Wing episode which discusses why we should, but somehow I think that wouldn't gain me much here. Discussions of the nature of man, and the establishment of wonder being particularly squishy in hard science terms.
Instead I'd point out that all safety critical systems are engineered around the notion of redundancy. Shit happens, and when it does, things break down. When that unexpected thing happens to our Earth-bound ecology, what, exactly, is our safety strategy? Hide in a hole? For how long? What if it's biological? What happens if someone accidently creates Card's molecular disruption device. We can't reasonably colonize another star system (yet) but we aren't *that* far from being able to establish some very worthwhile planetary redundancy. It's worth it because we are stuck on this rock that I think we should rename 'The Single Point of Failure'.