As a residency selection committee member for eleven years, and as a PD for one year, I've got an informed opinion on this topic, but keep in mind everyone has their own opinion and they may not agree with me (just ask my wife). Letters or emails that state "I plan on ranking you highly" mean absolutely nothing. There is no use in sending such a letter. However, a letter or email that states "I'm ranking you number 1", says a lot. I/we really do appreciate that.
Everyone wants to be happy with the Match. In my opinion, having a resident that really wants to train with us is more beneficial than having a subjectively slightly better candidate that doesn't want (as strongly) to train with us. So, I would disagree that a letter that states "I want to train with you" is of little value.
I'm a rather direct person, so why beat around the bush? Just tell your number 1 that they're your number 1.
Don't expect programs to tell you much in return, however. That's because it is impossible to tell how far down the program will go on the ROL. With 12 residents, a program could fill at #30 one year, but go to #90 the following year to reach their 12th spot. If a program tells you "we'd love to have you train with us", then I think its likely you're very high on their ROL. Other than that, it is exceedingly difficult to know.