I drink a lot of coffee and hate to waste it. Sometimes (often), I will get busy and come back and it is cold. Instead of throwing it out, I pour it back in the pot. I come back in a few minutes and have a hot cup.
The warmer is always on and there is no need to waste the microwave energy. I drink it black now, but used to fix the whole pot with cream and sugar. Works either way and there is no waste.