Doesn't the LOI depend on if you have offers elsewhere?
If you don't have any other offers, then it's obvious (or will be later in the cycle when schools can see where you've applied and if you have offers) that you'll go to the school that takes your name off the WL and offers an acceptance.
Without any competing offers, it seems like a letter of intent is more like a letter stating your interest in a school and a plea to move you up the WL. I suspect a lot of the action in this area happens in May when applicants are only allowed to hold one acceptance at a time.
If you publish an article, receive a significant award, get new MCAT scores, or do something else noteworthy that an adcom may be interested in, you could send a letter updating your application. Such a letter could move you up a WL depending on the school.