There are three things, really. 1) LOCATION
For a certain very narrow category of activities, the surface of Mars really is the superior location. It is better than any O'Neill habitat for this very narrow category of activities, whether that O'Neill habitat be in orbit about the Earth, in the Main Belt, or even in orbit about Mars itself. And what is this very narrow category of activities? THINGS DONE ON THE SURFACE OF MARS Well OK that seems obvious, but it's also true. If your goal is to explore the lava tubes of Olymus Mons, then Olympus Mons is superior to every other location in the Universe for accomplishing this goal. The surface of Mars is a superior location for Martian tourism. The surface of Mars is a superior location for exploring the surface of Mars (well duh). The surface of Mars is a superior location for searching out evidence of past Martian life. The surface of Mars is a superior location for living out one's dreams of being on Mars (well duh, pt. II).
2) SCIENTIFIC VALUE
Each planet in the Solar System (and many or all of the moons and a goodly number, I suspect, of the asteroids and comets) offers scientific discoveries that can't be made everywhere else. Mars has scientific value that 25143 Itokawa just doesn't. Then again, Venus has scientific value that Mars just doesn't, and Vesta has scientific value that Venus and Mars just don't. Yes, Ceres alone could provide more living space (and better living space) than Mars, Venus, the Moon and Earth all put together, but Ceres won't solve the mysteries of Mars (or Mars those of Ceres).
3) IT'S, WELL, **MARS!** HOW COOL IS THAT?
H.G. Wells didn't write about invaders from Habitat 212, Edgar Rice Burroughs didn't send John Carter to Bernal 36, and Ray Bradbury didn't write "The O'Neilian Chronicles." Mars has a history in the human imagination that no man-made object can match. It would be possible to dig tunnels which would be much more comfortable than the Lascaux Cave, and use some sort of modified ink-jet printer to put replicas of the famous cave paintings on the wall of you tunnels. But it just wouldn't be the same as standing in the Lascaux Cave and gazing at works of art sixteen thousand years old. Mars also has natural wonders which it will be difficult to replicate in habitats for some time, and again, it won't be the same as a NATURAL wonder. As wonderful as Island Three will be, it won't offer Vallis Marinaris or Tharsis.
These things Mars offers, and these things are reason enough to have people on Mars. Please, Mars advocates, don't try to argue that Mars is the perfect place to grow food for people mining the asteroids. It isn't. Nor is it the best place to get water, or iron, or anything else that isn't going to be used on Mars itself.

> If your goal is to explore the lava tubes of Olymus Mons, then Olympus Mons is > superior to every other location in the Universe for accomplishing this goal. Very true, but we've shifted topics here. When discussing O'Neill habitats, we're discussing space settlement. Your valid points are in regard to space exploration. Yes, granted, the points that O'Neill made in no way are an argument against the scientific explorations of Mars. They don't even rule out that some Mars explorers might chose to live permanently on the surface. But I think they may point to Mars being economically of much less interest than many currently suppose. > The surface of Mars is a superior location for Martian tourism. True, insofar that Mars becomes a locus for tourism. With travel times of a year or so, I would think it would be a much more limited kind of tourism than we might have of orbital or lunar locations. > These things Mars offers, and these things are reason enough to have people on Mars. > Please, Mars advocates, don't try to argue that Mars is the perfect place to grow food > for people mining the asteroids. It isn't. Nor is it the best place to get water, or iron, > or anything else that isn't going to be used on Mars itself. Andyou give very good advice to the Mars advocates here.

> > The surface of Mars is a superior location for Martian tourism.
>
>True, insofar that Mars becomes a locus for tourism. With travel
>times of a year or so, I would think it would be a much more limited
>kind of tourism than we might have of orbital or lunar locations.