Image from the Mars Curiosity Rover appears courtesy of NASA
I've had several conversations with people that don't see the point of sending a Rover to Mars. The criticism I've heard most is "what if we don't find the signs of life that we're looking for? Will the mission still be worth it"?
Curiosity is essentially a full laboratory on wheels. It will analyze everything it finds, so we will learn more about the surface of Mars than we have ever known. As we yearn to understand the beginnings of the universe and where we all come from, we need as much information as possible, even if it doesn't seem obvious at this point. The knowledge will lay the path for future generations to explore the universe in ways that we could only dream of.
NPR has a great article entitled "Science is Sometimes Wrong, For All of the Right Reasons". From the article:
In order to move forward, a scientist must have the courage to take the risk of being wrong. You stick your neck out so that you can perhaps see a bit farther than the others.