I'm glad it's not a necessity. If you guys look back at where the science of the planted tank originated, it was from a German book called "The Optimum Aquarium" published in English language back in 1988. I read this book in 1990 and at that time it was too incomprehensible to me (I was in 10th grade). Much of what is practiced today is based on that book, which was the first of its kind.
The concept of the substrate requiring its own heat source comes from the idea that water does not circulate so well 3 or 4 inches under the substrate and so if the room ambient temperature is cold, it might eventually cause problems. Remember this book is from Germany, so those of you from Florida or Texas would never encounter this problem.