Well half of the moon is dark to us, because of the moon's orbit and rotation on its axis the one side always faces the earth and the other into outer space. When we see a full moon on the eearth, that side is completely dark.
I don't think you are grasping the enormity of the moon, that it would be nearly impossible to "strip-mine" the moon into a dark shell of its former self and no-one is suggesting that this happen either.
Part of the great thing about technological progress and reaching out into space especially is just how much it DOES benefit the human race as a whole. Capitalism and greed and even war has been largely responsible for the high standard of living we do have at the present time. The space program has been responsible alone for thousands of inventions and new methods that have improved human life just in the past 40 years. I mean, think of it. Out of all the ages in human history, which time would you rather be living in? For me it would be none other than the present day. The problems that we have cannot and will not be solved by throwing every last dollar, resource and bit of human energy at them, I believe our only chance is to constantly progress and focus our energies on expanding our knowledge of the universe.
And in terms of "ruining the earth", it helps to take a more balanced approach on just how our earth is being ruined by seeking out many sources on the subject. Remember that scientists are also not immune to the HUMAN feeling of greed, large corporations don't have a monopoly on it. If saying something will get a scientist more grant money to continue his or her reasearch for another 5 years then you bet they will say what they need to say to do it. Some might say that we have made strides in pollution control in the past few decades. No, we aren't pollution-free yet, but reports of us being on the brink of global catastrophe are a bit exaggerated.