Definitely alternate with at least one other type of workout. Even though I LOVE kickboxing, I know the benefit of cross training (reducing injury risk, as Suzanne mentioned, as well as working the muscles in a different way, as well as not letting your body adapt to any one form of cardio and making sure you don't plateau) so I alternate kickboxing with step, hi-lo, rebounding, etc.
Edited to add: You COULD do step as you only cardio, if you put one day in between your cardio workouts, and only do it 3 times a week. But it would still be beneficial to cross train.