I think

the argument is more about whether a manned-mission to the moon is necessary.

There isn't a single thing that a man can do on the moon that a robot couldn't do, at least from a scientific perspective. The difference is that a robot is significantly cheaper to put on the moon and would bring back the same scientific results, at least in the long-run. The primary argument for manned travel is that a human can accomplish more on a single mission because they do not have an extremely specific function, unlike a robot, so any information could be acquired at a faster rate. Many scientists would just argue that you could send more robots with different specialized functions and you would get the same (and even better) scientific data and it would be significantly cheaper.

NASA's budget has been tight lately. Obviously the budget was huge during the space- race (over $33 billion when adjusted for inflation at one point), but as of now its budget is fairly meager, sitting at 0.55% for 2009 - or $17 billion. They have been less well funded in the past, for example, in the 1970s, 80s, and later 90s. The pinch is starting on them again. The problem is NASA has expanded their scope over the last 20 years beyond just space (partly because they were federally mandated, partly because there were political pressures, and partly because NASA just wanted too). They now research climate, atmosphere, and natural terrestrial phenomena like hurricanes, they've gone into biology research, water technology research, medicine etc etc. That costs a lot more and they aren't seeing the funds.

Now to my conjecture why they want to keep manned space travel so badly. The primary reason is because it inspires awe in people and public opinions polls show strong support for it. If they were to eliminate their manned-programs, I believe they fear it would be easier for politicians to cut their funding even more since their constituents wouldn't really care about the program.

Also, NASA does need to be better funded because science and technology are easily the best returns on government investments in the long run; both from an economic perspective and just an achievement perspective. It creates a lot of jobs and the technologies produced are a huge economic boom. There are so many commercial technologies that are apart of our daily lives that came directly from NASA, and we don't even realize it. Just to explain how NASA research enters our lives in seemingly unrelated ways. Remember those ridiculous swim suits at the 2008 Olympics that Michael Phelps and everyone else wore that produced those amazing times ? That was NASA right here.