Fine, but by a similar arguments space exploration as a whole, manned or unmanned, is basically worthless. Learning about Martian soil is unlikely to, in and of itself, provide any economic or "practical" payback to those of us here on Earth in the utilitarian sense you seem to advocate. Instead of worrying about space, we should allocate our money to things with more immediate and predicable impact; there's no shortage of opportunities in medical research or technological development in energy, transportation, or computing where money could be better spent by your measure.
Yet, clearly adventure, wonder, art, and exploration are highly valued by most people--a huge portion of our privately funded economic activity is directed towards these things which have seemingly no economic value, and has been throughout recorded history. And just like more "practical" areas, sometimes there are projects so big and so risky that no single private entity can take them on, but that doesn't mean they're wasteful. You don't here anyone say that they sure wished the Byzantine's didn't waste their resources building the Hagia Sophia or the Americans the Apollo project, because while there's no obvious practical benefits to either which couldn't have been met more efficiently though direct spending towards practical applications, both had immeasurable cultural value.
I feel the same way about manned spaceflight today--the only good reason to explore the universe is for the adventure, because it's exciting, interesting, inspiring, and enlightening. In short, because it makes us better. And I see no better way to meet that objective than through manned spaceflight.
Expecting space science, whether conducted by robot or man, to cure the sick or increase economic productivity is like expecting to buy a winning lottery ticket. So if science is your only justification for space flight you shouldn't expect space flight to last much longer because we can do "better" science for cheaper right here on Earth, and there's no shortage of ideas to fund.