The Illusion of Extra Credit

My Criminal Procedure professor has given the class the opportunity to write a short paper for extra credit. But in an environment where grades are curved--that is, where the students are graded against each other, not the test--can there be such a thing as extra credit? If my neighbor does the assignment and I do not, surely she won't just drive her grade up, she'll drive mine down if she passes me in the class ranking. I have to do the assignment too, just to keep up. For this reason, anything labeled "extra credit" smacks of graded work in disguise.