Actually, RI DOES require all boats over 14 feet to be registered, motor or not. Anything with a motor, no matter how small, must also be registered.
Of course, no one enforces this for small sailboats (CG, local harbormasters, etc.), which is why I was the only one in my fleet for years and years with registration stickers on my H16.
Registering them actually helps when it comes time to sell, especially if to someone in a state that is more enforcing. Otherwise, you're in a paper chase for a cert of origin, etc. One good thing about Hobie Cat is that this can be obtained fairly easily through a dealer, although it can take time.
Mike