According to an email I received from Jerome Guillen, VP of Sales & Service, if you bought the pre-paid service plan you can use those pre-paid service visits at any time. They don't have to be annual, and they don't have to conform to any mileage intervals. Dr. Guillen said those are merely suggestions, but that owners can essentially do what they wish.
Tesla keeps your car for the entire day when you go for annual service. The value in the annual service is the detailed inspection that Tesla conducts of your vehicle. This is not just a "courtesy inspection", which they will do for warranty work. One of the items they performed, which I hadn't expected, was that they took apart the underbelly of my car to check and re-torque all of the suspension bolts as needed.
I bought the service plan because I felt there was value in having the car closely inspected. I'm not the type of person who has ever done any work on a car, and given the newness of the vehicle design, I didn't think having it inspected annually was a bad idea.