Having a space station is not important. Having the infrastructure to build and maintain a space station is important.
When the US stopped sending people to the moon it stopped to be capable of sending people past LEO. A space station at least ensures that we can still send people to space.
But we (USA) can't still send people to space. We voluntarily gave up that infrastructure years ago and have no clear, funded plan to restore it.
For manned spaceflight, NASA's top priority should be basic research into breakthrough propulsion technologies. It's pretty clear that we'll never be able to safely send a large number of people anywhere worthwhile using chemical rockets.
We do have a funded plan to restore it. SpaceX and Boeing have contracts to ferry people to the ISS, starting in a year or so. Both of them have constructed serious amounts of hardware, both the capsules and the pads/towers required to launch them.
China is replicating exactly, step by step, the soviet space program, and the next stop is their own MIR. There is a lot of technology to be learnt there about maintaining medium-term live in space and basic science research that could be done.
Well, there definitely is some value in studying the extraterrestrial space medium. Regarding the ISS, just think about its sheer size -- how else can the knowledge about constructing and maintaining such structures be gained? Besides that, a lot of knowledge and experience was gained or confirmed/replicated from previous experiments. So the answer in two words: research & development (with emphasis on the last one).
