Seems to me that you're presuming that we cannot attain information about Mars without actually putting a man on the planet. I don't think Columbus is a good analogy - 15th Century Europeans had no way of discovering what was out there, sailing west. We, however, have landed several Mars lander missions that have gathered quite a bit of data, taken images and video, and brought back samples. I'm just wondering what it is that a manned mission is going to "figure out" that we can't otherwise do. If there's something, then perhaps it's worth it. I just don't see it at this point.
(I'm not questioning your conclusion - it's a matter of choice. I'm only questioning the premises stated).