"But...has anyone come up with a great answer to Sean O'Keefe's recent question about Mars, namely why should we go? If we set foot on Mars, then what? What does that do for us?"

I think same question can be asked about the whole governmental agency NASA, or even the whole friggin US government.
The correct answer is that is to serve the people [and no, not to eat them:].
NASA is exploring space for the people of United States. Exploring means looking for something which MAY have value- not merely wandering around.
Now an entire planet fits into my definition of something which may be valuable.
Ok, so say we find a mountain with gold in it, or the State of Alaska before there was much economy activity there. Does it really have value to us at this time or within a "reasonable" time period. [A reasonable time period for a person is generally a much shorter time period than for a nation.]
The question for NASA is: is there anything in space which has value for the American public within a reasonable time period.
NASA does not have the option of answering "No, there isn't" . If the answer is "no" then we the tax payer would be foolish to continuing paying billions of dollars each year for something which has no value.
So, getting back to the question, why send crew to Mars? The answer is that some of the people in your nation within a couple decades will live on Mars. Not because they are transplanted by the government to live in some strange zoo, but for the same sorts of reasons people live anywhere on Earth.