It's not for the science. We get more science for less money and less risk out of robotic missions. The Russians did robotic sample return from the Moon back in 1970. The Mars Exploration Rovers have been working since January 2004 - coming up on four years. Our orbiting satellites do more science than the overworked crew of the ISS has time for.
I don't even know that I buy the "emergency lifeboat" argument - that we must spread our genes beyond Earth to survive some possible cataclysm. It would have to be a hell of a cataclysm to make Earth less habitable than anywhere else. Like, Earth reduced to a fine powder, or ever square inch irradiated to a deadly degree. Fixing whatever hypothetical damage might be done, even by an asteroid strike, would almost certainly have to be easier than terraforming somewhere like Mars. Not only are air and water lacking, but our molten metal core - which produces our magnetic fields and keeps Evil Space Radiation from killing us all - can't be easily reproduced. And never mind that we're evolved to live in 1g.
We want to go into space because it's wicked cool. It's a grand adventure. It stirs the spirit and lifts the hearts of many.
"Should the government pay for it" is a separate issue. The government pays for public art and monuments and other such things that serve no practical purpose, but which we as a society hold to be important enough to merit funding. The great adventure of human space flight, and the national pride that goes along with it, could possibly be considered a similar investment. (And it does serve at least a small practical purpose, in that it keeps current the skills of a segment of the aerospace business which can be turned, lickety-split, from dreamy civil to hard-nosed military ventures.) Or maybe it's not the government's business, and the new space entrepreneurs should be allowed to lead the way.
But before we can really debate what the manned space program is worth and who should pay, we have to be honest about why we're doing it. And it's not for the science.