I think I see what you're getting at, that as far as sheer scientific return for the dollar, robotic missions are much more productive. And I agree, for the cost of a single manned mission, several probes could be launched to destinations all over the solar system. For the cost of a very modest manned spaceflight program, we could have a truly extravagant program of exploration and discovey using robots. Personally, I feel we have over-emphasized our manned programs from the beginning, and it has hurt all our space exploration efforts as a result.

But space exploration is more than just about sheer practicality, there is a very powerful spiritual and human component, something the race needs at a fundamental level to maintain its long-term survival. To give up on manned space travel entirely and rely solely on automated probes would be like abandoning astronomy and physics and sinking all our money into medical and industrial research. A short term practical benefit perhaps, but an abandonment of all future progress and discovery. All of the great advances in science have come from totally unexpected and unpromising research done by wild-eyed dreamers with no hope of any "practical returns on investment". Managed, applied research just gives us new, improved consumer goods.

Although I can't prove it, I am convinced that someday our automated spacecraft are going to find something out there that is going to demand our immediate, personal attention. We had better be ready.