I don't think interval training is what people are talking about when they say cardio is a waste of time. I think they're talking about the hours long monotonous biking or jogging. Interval training is anything but monotonous.
If you're going to call interval training "cardio", then you might as well call strength training cardio as well, they're two different categories(hell, scientists are saying that guys looking at boobs is a sort of cardio workout). Haven't you ever heard fighters talk about conditioning, conditioning, conditioning? Conditioning is key? Interval training is basically what fighters do, called "conditioning"; more like sprinting, with short bursts of all out effort. Por ejemplo, a boxer would go all out on the punching bag, then jump rope for a while, then go all out doing pushups, then jump rope for a while, then go all out running laps around the ring, then jump rope for a while...that's called conditioning, or what you guys are calling interval training.
The monotonous exercise dubbed "cardio" goes by no other name but cardio, wherein I think the miscommunication lies. In my opinion, that "cardio" is a waste of time for any healthy individual. Only maybe if you're severely obese or have slight heart problems, you may want to start off there, then move on to strength training and conditioning when you're ready.
