The risk in not signing the letter is you could lose your spot in the recruiting class. If you commit but never sign a letter, then the coaches have to figure you are not totally sure and they will likely continue recruiting other kids. If you're a basketball player and don't sign in November, but commit to a school, you run the risk of injury, a subpar season, or something else kind of short-circuiting your college intentions. A signed LOI locks you into the school in the fall, they will quit recruiting at your position, you have a spot and are locked in with little to no risk, etc. Almost all schools will honor a LOI if signed in the fall (assuming no legal issues arise). But, if you don't sign, you leave yourself at the mercy of your senior year performance, injuries, other players becoming more attractive than you, etc.