Forget about costs , public and political opinion for a second ( only a second mind you!). Sure we have gone to the moon and walked on it, but we have never lived there long term. there are many many challenges to learn about living on another celestial body. they say the last time, the biggest problem was dust. sure they prob have systems to combat the dust now, but wouldnt it be better to test this close to home, where rescue is possible? Dust is just one issue.
Also, nothing brings problems to light than actually going out and getting your hands dirty under real life combat suituations. suppose some of those problems are serious, and threaten the safety of the astrouauts.
there are loads of other issues that will have to be tackeled and tested under combat suituations and I would be much happier if the astronauts doing the testing had some hope of being rescued should anything go wrong.
This also makes sense from a publicity aspect. suppose we send folk off to Mars, they make it there and something catastrophic happens and they all die as a rescue would take many months. Public opinion of travel to other planets would be blackened for many years and could set us back a long long time. However if the same suituation happened on the moon, but we could mobilize and mount a rescue in a couple of days.
The astrounauts would be much safer starting off living on the moon.
Should anything go wrong, chances of rescue are much higher.
"Success is the happy feeling you get between the time you do something and the time you tell a woman what you did." Dilbert.
  