I've been so on the fence about sending an LOI, particularly post-II pre-waitlist, and given the rather disparaging light in which sdn seems to cast them. Obviously some schools soft require them, but I'm not sure if the fact that a school accepts them means they have a lot of (or any) utility. It seems as though not withdrawing an application is message enough that you're still interested.
That said, I do see how at a non-rolling school, there may be some utility in expressing that after, presumably, other interviews and acceptances, you're still committed to attending that school upon acceptance. It's difficult to say.