And I've never really understood why this causes problems. Do people really get these emails and think, "Oh man, they really want me! I'm going to suicide match and rank only them!"
I guess maybe people would bump a program up on the rank list based on "feeling wanted," but even that I don't fully understand. If you've got your programs ranked 1, 2, 3, and 4, and program 3 says, "we're ranking you really highly," you don't have to move them up in order to match there if 1 and 2 don't pick you.
That's the beauty of the match. You rank the places in the order you want to go there, and the programs do the same. The highest place that picks you and still has places is where you go. You get the highest program on your list that has a spot for you based on where they ranked you. There's really no way to "game" this to your advantage unless you really think that you can influence where a program ranks you. And keep in mind the programs have all been doing this a lot longer than you have. I can't imagine PDs would be so easily duped into moving you up their list just because you told them you ranked them highly. Why would fall for the same thing from them?