First let me say that kickboxing is not for self defense it is a sport. There are major differences between sports and real self defense.
As for the workout and the validity of the technique in a cardio kickboxing class it depends on the focus of the instructor. We teach cardio kickboxing in such a way as anyone that wanted to work their way into the ring they could.
The basic focus of cardio kickboxing is to use the kickboxing training and drills with cardio conditioning as the main focus. Tae Bo is a very good workout but the way the techniques are performed they are pretty much useless for use in the full contact arena.
In our case the skills taught and the drills used are to foster correct technique, correct kinematics of the motion, and correct power generation. We use shadow boxing, speed bag, heavy bag, jump rope and calisthenics’ in a random order to constantly shock the muscles into being challenged throughout the workout. For stamina drills we ramp techniques with consistent correction to achieve proper execution along with the ability to work through several rounds without the muscles tiring.
In intermediate level class we introduce light contact training along with more concentrated endurance and speed drills.
We also have an advanced class for those that do want to do hard contact sparring and possibly enter the ring.
For those that do want to go into the ring there are personal training sessions to prepare for each fight and they are also expected to have a personal training regiment that includes personal weight training regime, road work, correct sleep and diet.
We have several fighters that have fought in the IKF circuit and have done very well. We have had many more people chose to stay in the basic class because of the great workout and that is all they want.
But remember - This is still a sport/physical fitness exercise.
Self Defense is if you lose you possibly die. A totally different mindset and focus.