I agree that for most people, 2-3 non-consecutive days a week is enough. More than that will increase the risk of overuse injuries (and make it hard to not do step on consecutive days). This is also what Cathe has been recommending lately when she's asked the question.
I'm sure that, as with most things, YMMV, but better to be safe than sorry, as doing too much of any one type of movement can lead to muscle imbalances and increased risk of injury. I think kickboxing is a great form of cardio to cross-train with step.