I don't thing there is any one-size-fits-all, Only-One-Best-Way to design an ongoing exercise program. Some prefer to do cardio and resistance work in separate sessions, others prefer to do cardio and resistance work in the same sessions, and of course there are hybrid circuit workouts that have cardio and resistance mushed together. And then there are fools like me who like the water too.
Rotations are good for keeping variety up; boredom (body-mind boredom) is a sure killer of consistency, and the same-oh week after week can take the edge off your improvement. They also help exercisers plan their upcoming weeks, making sure all the components of fitness (cardio, muscle strength, muscle endurance and flexibility) are addressed. Different rotations have different fitness goals built into them.
I personally don't follow rotations as they are put up here, although I've suggested a few. That is NOT to say that they don't work. They work very well for those who prefer to do them. It's up to the individual exerciser whether rotations will work for her, and you may like some more than others.
Plus - rotations sell DVDs. How bad can that be?
A-Jock