Exercise in any form - mild, moderate or intense - is good for you and can help reduce the risk of disease and keep the heart healthy.
But when it comes to strengthening bones, milder forms of activity may not be enough. This is why I stress on the importance of weight training. It doesn't mean you have to go and "bulk" up like men do. You can simply lift 5 lbs and still strengthen muscles. By doing this you can decrease your risk of osteoporosis!
So, yes, cardio is great. But don't skip out of the weight/strength training!