Correct me if I'm wrong but the Serious Growth Plan is only to be used for a limited time. As such it's alright to bend the rules a bit to shock the muscles. But for any long term lifting schedules working out while still sore won't amount to much progress. You need to let the muscles recuperate and repair themselves to acheive strength and size gains. If you are constantly working a muscle and not letting it heal you are defeating the purpose.
Remember, you grow outside of the gym not while working out.
If you want a more scientific explanation I'm sure someone on the board can help you out.