LOI need to be direct and honest. Don't hold any punches. Make it clear that you want to attend their school and if given the chance, you will attend. Be specific and keep the letter tight, to about one page. Don't cover ground that has already been covered in your application before.
And to redirect my thoughts about essay services, I think they can help because you get feedback from people that deal with essays for applications. These aren't your typical essays, because these people recognize that our readers will only give them a cursory reading. English professors don't know what our readers want and might be looking for. A good editor will keep your voice. A good editor won't write your essay. They'll only help you craft it. Getting into medical school is tough, so I think we should take advantage of every opportunity we can. Many people are against editing services. And that is because you can't be sure that you're getting a good editor, especially with the really large editing companies. So, find one you like and stick with him or her. If you have a friend that is good, then use that person.
But it can only help you to have an editor. It can't hurt. What's 70 bucks more, we're already spending thousands. The worse that can happen is that you don't like the changes. But I've always been happy with my editor.