If that's the school that you would be most likely to attend if you were accepted you might as well send them a letter of intent anyways. The only problem is that it might be hard for you to put your heart into something that may very well do nothing. If you aren't accepted anywhere and you are on another waitlist and you just want to get into medical school then it would be a good idea to send a letter of intent to the school whose waitlist is not "set in stone".