If you are guaranteed at least the category you are paying for, why doesn't everyone book a guarantee cabin? Many cruisers prefer to be mid-ship or near the forward or aft elevators. Others prefer either a higher deck near the pool or a lower one near the the main restaurant or casino. Using a cruise ship deck plan, they can choose a specific cabin on the ship that is in their preferred location. They know well in advance just where they will be sleeping! On the other hand, those who book a cabin category guarantee might get lucky and get an upgrade to a suite or from an inside cabin to an outside one.
Every cruise traveler needs to decide which is more important to them, and whether they want to gamble on a cabin category guarantee or select one in advance.