Should manned exploration of space be abandoned? 

The best answer to this question that I have heard was written by J. Michael Straczynski, for the first season of his television epic Babylon 5 :

A reporter is meeting with the head of the Babylon 5 station, Commander Jeffrey Sinclair, after a difficult day:

  "Reporter: "After all that you've just gone through, I have to ask you the same question a lot of people back home are asking about space these days. Is it worth it? Should we just pull back, forget the whole thing as a bad idea, and take care of our own problems, at home?"

    Sinclair: "No. We have to stay here, and there's a simple reason why. Ask ten different scientists about the environment, population control, genetics - and you'll get ten different answers. But there's one thing every scientist on the planet agrees on: whether it happens in a hundred years, or a thousand years, or a million years, eventually our sun will grow cold, and go out. When that happens, it won't just take us, it'll take Marilyn Monroe, and Lao-tsu, Einstein, Maruputo, Buddy Holly, Aristophanes - all of this. All of this was for nothing, unless we go to the stars."

(Text excerpted from The Lurker's Guide to Babylon 5)