The reason behind STS being a 3 day per week strength training program is that we believe three times per week is doable for most home exercisers and still leaves room for your cardio and other workouts.
In our opinion three times per week at a high intensity level, like STS delivers, is more than sufficient to fulfill all your strength training needs. We believe in a balanced and realistic approach to fitness that always includes strength training, cardio and stretching.
Some exercisers may choose to do each STS workout twice per week and that is fine, but they will not get anywhere near twice the results and run a much higher chance of mind fatigue and burnout. We feel that strength training three times per week will give you great results and still leave time in your busy life for other things.