The rotational system is mandatory as your server and all other diners will be moving to the next restaurant. Each restaurant has their own menu so if you didn't move, you would have the same menu for three days.
There is no dining plan on board as all meals and food is included. The only thing you pay for is drinks from the bar, drinks from Cove Cafe, snacks/drinks at the theater and some snacks from room service. If you chose to go to the specialty restaurants, that is extra also. Otherwise, all of your food is included.