i hate to second that notion but that's the fate we face on waitlists. you're there to serve the school's needs, not your own. they want to know you're interested only to guage how willing you would be to pick up your whole life and move to st louis at the end of august when a seat opens up. if they are in *need* of someone with your scores or find that their pool suddenly shrinks but their class isn't near full then they will pull us off the bench. it sucks and i completely empathize with you. (different waitlist but same agony). Def send the letter and remain interested but for what it's worth, for my own sanity, I got real excited about the school I've chosen instead and set a realistic cut off in my mind--if I don't hear by July 1st then I will commit completely to aforementioned school and decide to be happy because, hey--that's life.
good luck! and be excited about your other choices. if law is what you **really** want then be excited to start wherever you will be next year