At some level we need to continue space programs until we have permanent colonies elsewhere than Earth. This is from the point of view of protecting humanity from Earth-wide castastrophies such as large meteor impacts etc.
Now there are also lots of catastrophes which we need to try to prevent with non-space investment. World hunger, environmental impact, global-nuclear war, biotechnology mistakes, etc.
Also, I wouldn't put my money on NASA alone. They are severely limited in their approaches to solving space flight. This often happens in technology -- it is sometimes better to start again from scratch rather than continuing to upgrade older stuff. There are actually a number of exciting private ventures looking at re-energizing space exploration. See
http://www.msnbc.com/news/904842.asp?0dm=N16LT.
Of course, space flight will always be dangerous and expensive. So for the average person, investment in space exploration will help little in the things that matter (health, environment, etc.). For example, if a meteor was about to crash into Earth what are the chances of any of us individually getting to escape to Mars? It will be a priviledged thing for a long time to come ...
Space flight is not and will likely never be very safe. The reliability of any machine is related to its complexity -- the more complex, the less reliable. Furthermore, any failure is likely to have fatal consequences. It is very likely that there will be a Columbia/Challenger type disaster every 10 years or so.
Overall, I would personally prefer that we concentrated on making Earth a great place to stay, rather than trying to figure out a way to leave it. Only in order to stave off uncontrollable natural disasters do I support large government investment in space exploration.