FC letters are not “necessary:” no BS requests one or even mentions them. I think PhotoMom is spot on–if you’ve applied, you’ve told a school you want to go there. FC indicates you think you will be one of the lucky ones with more than one choice, or you’ve been foolish enough to tell one school the other schools you’re applying to and where they rank (indeed!). In either case, FC letters smell of strategy, hubris, or desperation.
Also, do you really think the admissions offices haven’t seen a million of these? I’m pretty sure they’ve also seen enough FC applicants who didn’t yield to know they aren’t even reliable. You’d might be surprised at how many accepted students have a different FC after revisit days if they are fortunate enough to have more than one choice. When the tables are turned, and the schools are courting YOU, they begin to look and feel very different.
Remember, boarding schools make their first-choice picks based on their needs and use their waitlists to fill any holes. They’re OK with less than 100% yield as that it not their main goal. They have many years of yield management experience, and I doubt they rely on FC letters for improvement or they would request this info up front, not hope for the most savvy to figure out the subliminal message they are (not) sending.
I do agree, though, that from a waitlist, a FC letter indicates continued interest and may help the AO select among many to fill a hole.
Good luck!