I'm no expert, but I'd really recommend a "Why Harvard" that mentions specific programs and professors. Poke around on the web site, read the publications, examine the special programs, pick something you like and mention it.
"My main interest is Issue X, and I believe Harvard's program on X is the best in the nation. I particularly look foward to studying with Professor Y, whose recent publications in Journals A and B show why he is a leader in the field."
Something like that, but better. That's what I would do. Be prepared to back it up if you get a call -- it will help if you really mean it and you have the resume or academic history to prove your interest.