But cardio IS a waste of time if...
a) You are doing a lot of it and you are not getting results. That goes for any form of training. But every week or so I get an email from someone doing mega amounts of cardio (in some cases, 7 hours per week!), but the person goes on to complain they are not losing fat even when doing this much exercise.
So clearly, this is a waste of their time. What's next? Quit their job? Workout 3 hours a day? Forget it, you need something besides cardio in this situation.
b) Cardio causes an overuse injury. This happens all the time. If you are a runner logging more than 20 miles a week, chances are you've been to the physiotherapist, doctor, or chiropractor because of your high-volume running. But that's what happens when you do something thousands and thousands of times per workout. You must expect overuse injuries. But a good program should not leave you injured. If your routine leaves you with medical bills, it's a waste of time.
c) Cardio is a waste of time if you could be getting more results (or even the same results) in less time. Let's say we take 2 guys, and each can find time to exercise about 3 hours per week. Guy A runs 60 minutes per day. Guy B (they're both French, that's why they are both named Guy) does 25 minutes of free weight and bodyweight training followed by 20 minutes of interval training.