For those with borderline stats (like myself), I believe this is an absolute must! It cant hurt, and we need any small advantage we can get.
Think of it as another way to get your name out there. With all the applications they looked at, interviews they gave, its easy for you to get lost in the mess. If two weeks later (around the time theyre making decisions), your letter pops into the office, it will automatically make them think of you and what they liked about you in the first place.
I was 2 for 2 with thank you letters. Did it make the difference? Ill never know, but it was worth the chance.