One word: conquest.
If it weren't for that drive, we wouldn't bother to go at all. At this point, and for the next hundred years or more, we're going to do nothing more than a brief visit. But putting people on the surface - that's a Fuck Yeah! moment. People won't pay for robots, but they'd all chip in if some dude went and wrote his name in Martian sand in pee. *shrug*
BTW - I agree that robotic missions make much more sense.The manned program has always sucked NASA dry on a year to year basis, but if you ask the average person to name an astronaut and a robot mission to a planet, you'd better believe you'll get Neil Armstrong and a blank stare. (There's an outside chance you'll get Voyager, but my money is that half of them will call it Vygr)