Agreed...letter focusing on PD and chair is good.
Thank you cards are a waste of ATP molecules and a waste of money/postage.
I have even gotten thank you cards in the past...which is nice of people but I know it's part of the game. Somebody decided to perpetuate some myth a while back saying that thank you cards to all the interviewers were good. Then some sadistic punk decided to step it up further and insinuate that not sending thank you cards would hurt you because everyone else is doing it.
Please please please let's not start with the holiday cards

Don't be that guy!