If you're talking about the philosophy of Dead Orbit, it's not about running away. It's about diversification of risk. If we put all of our eggs in one basket, our enemies only have to tip one basket to break all of our eggs. We saw it happen with the Cabal. If all of humanity had been in the City during the attack, we'd have been wiped out. We survived and struck back only because we were spread out. The same logic applies on a larger scale. If Earth is our only permanent settlement, we're screwed if it gets destroyed. If we colonize other worlds as well, we have a better chance of bouncing back from planet-wide disasters.