Another philosophical aspect of the question is to what degree are humans really "there" through the senses of our robotic creations? In some sense--maybe a very deep sense--our machines are prostheses that extend our bodies and senses. I think I can argue pretty credibly that humans are already on Mars, and have flown by the outer and inner planets, through these mechanical extensions of ourselves. I find that satisfying right now because I played a very tiny part in the current MER missions through NASA's use of some image processing software I wrote to clean up MOC images for site selection. There's a tiny virtual spark of "me" on Mars right now. Maybe once removed--the software was an extension of me which extended into the MER project.

humans could also go there, though perhaps not as much for the purpose of knowledge as for the reason of personal challenge.

I'd like that, being able to honestly admit why we want to go, and not resorting to drab bean-counter arguments like spinoffs and orbital manufacturing, et al. If we resolutely take the pragmatic view of space exploration, then I think we're not going to experience the full spiritual effects of it. Space exploration is as much about the soul as the bottom line.