are they necessary if you are going to write the PD in a few weeks to let them know where you intend to rank them?

One chief resident told me that they frequently do get e-mails and letters informing programs that applicants intend to rank them number one. I would reccomend that you do this for your number one choice, I can't imagine that it would hurt your application. I've heard that many programs just throw these letters/e-mails out, some will take the time to respond to them, and some are rumored to use them as part of their criteria for ranking you because it's considered prestigious for a program not to have to go too low down it's rank list in the PD community and an enthused intern is better then one who ended up getting his last choice. I wouldn't call it "necessary" though, I would just call it as one of those things that probably won't hurt to do.