I don't remember where I heard it, I guess from a classmate. I understand that you wouldn't type a Xmas card VentDep or what not but this is business and in my letters I am trying to convey the level of interest I have in the program, where I will rank the program, why I liked the program, how serious I am about the program, as well as a thank you and a personal note to the interviewer. It would seem weird for me to do all of this on a little card in my horrible handwriting and have them use this to evaluate me later in the ranking process if they choose to do so.
But I am a pretty serious guy maybe someone less serious can show their more relaxed nature about work with a card, I don't send anyone cards even for birthdays or Christmas its sort of not in my personality... that's my take on it. It would be forced and not as authentic as a truthful sincere letter... for me.