Prefab Programs
As a current personal trainer, exercise physiologist, physical therapist and certified athletic trainer who has worked in the fitness industry for over 23 years, I can tell you that A-jock is correct in her contention regarding specificity of training.
Yes, there are some cardio conditioning benefits gained with certain types of strength training, some strength gains seen with certain types of cardio training and flexibility improvements that can be gained with either type of training, depending on the level of fitness of the individual when beginning a program. However, the best improvements in overall fitness come from programs that include some of each type of training.
No one canned program--be it Body Rx, Body for Life, etc--will provide individualized programming. Are they better than nothing at all? Absolutely. They also, by coincidence, may provide a better choice for a given individual than the program the individual may have designed for him/herself. But this doesn't mean that the program is right for everybody. And, just as with fad diets and eliminating an entire category of food, any program that suggests eliminating an entire category of fitness training from a program is a mistake. The hype sure sell a lot of books, though!
The percentage of exercise time allotted to a particular type of training depends on individual physique differences, needs and goals. There is no one perfect program, nor does one size fit all. If you're satisfied with the results of a particular prefabricated program, great! However, realize that this won't be the key for everybody. The best program is one designed for the individual and that the individual will follow.
Maribeth