The discoveries made by Opportunity and her twin Spirit have been essential to our current understanding of the chemical makeup of Mars. Even so, 10 years into its mission the rover discovered evidence of clean, pH neutral water for the first time, making it even more likely that the red planet once had life.
Apart from the invaluable information it's brought us, Opportunity has served as an inspiration to young engineers everywhere, including myself. No environment can be tougher than the surface of an alien world, yet it has somehow lasted 40 times longer than it was designed to. If our government chooses to leave this perfectly functional machine to rot on the surface of Mars, we will be an embarrassment to the scientific community, and send the wrong message to our young engineers.