There is actually a difference between letter of interest and letter of intent. To clarify, a letter of intent is something that you send to a school post-interview where you commit to attend that school if granted the privilege of admittance. A letter of interest is something you do to demonstrate continued interest in a school (this typically occurs when you are on a hold or have not heard re: interviews and includes grade updates).
I sent one letter of intent out and I do not know if it will help. My interviewer at the school recommended that I do it to school that the school is really my first choice because they want to match the right students with their school. I'll let you know how it works out.
But if you do send one, make sure it is well-written, it will go in your file for the ADCOM to see. 🙂 Good Luck!