Part of my job is putting together AAPs (Affirmative Action Programs), so maybe I can help.
For our clients, they are required to give us the race and sex of any applicant, whether or not they are called in for an interview. When it comes time to update their plan each year, we have to count each and every one of these applicants for that particular position. If you have 100 applicants, and 50 of them did not provide the employer w/ their race and/or sex, it really skews the results, as we can only then count the 50 employees that did provide this information.
So, please fill out that card and send it in! You will be doing the company a favor.