The primary purpose of a Mars mission would be to keep the public excited about space science (and thus willing to fund it) for the 15 to 20 years it would take to get there. During that time huge advances would be made in the private sector, and hopefully significant strides towards a space elevator. Because without a space elevator (assuming no unexpected breakthroughs in propulsion like a Higgs Field inertialess drive) spaceflight will remain too expensive for any real progress off the Earth. Chemical rockets simply don’t have the energy density to cut it.
I might add a Mars mission would not be as useless as cedarford sneeringly comments. (One suspects cedarford makes his living off unmanned probes. Such people tend to be strongly against manned spaceflight as it “picks their pockets.”)
