As we use up the natural resources on earth, turning to space to replace lost precious metals and even supply water is going to be essential. Initially, yes, it will cost billions of dollars to be in space, but mine one single SMALL asteroid and we're talking about working on a profit.
See: Mining the Sky by John S. Lewis
http://www.amazon.com/Mining-Sky-Untold-Asteroids-Planets/dp/0201328194
Turning our backs on the potential of space is like sitting, starving, in front of a feast that is ours for the asking.
The *problem* is that space exploration only makes sense if it is privatized. NASA works too slowly and has to deal with government funding. Only privatization with people investing in space travel and expansion is going to make it progress. When was the last time someone walked on the moon? Do you even know?
Right now the Chinese are making much more progress towards space travel/expansion than anyone else. Paying attention to earthly dynamics means that their foothold in space could actually be dangerous for those of us who are letting go.
Is space travel necessary? Yes, I believe so.
Is it profitable? Without a doubt-- and we have the technology now.