Mitch, I live in the city and have made good use of my space. My pond is only three feet deep and that is just the way it is. Even if my pond was 6 feet deep, that does not mean that you can just leave a heater in and all will be well. The Local Pond Guy in town has a really really big 6 foot deep pond and he lost lots of his fish last winter with just a heater and a bubbler.
I think the trick is to have running water in front of the heater, that way it will evenly heat the pond better. If you just use a heater, you will only have one small opening in the ice, and then the fish will hang out at the heater and things go bad then. But if you have running water in front of the heater, you will have more than one opening in the pond and the fish will not hang out at the warm heater.
Mitch I have seen many people's ponds fail over the many many years that I have been ponding it. Those who listened to me did much better over the winter, and those who listened to the so called "experts" have suffered and many have lost all their fish. I can tell you many stories over the many many years of things that have gone horribly wrong for many people....
Running water all year round is ideal for optimal fish health, and combined with a heater over the coldest days is ideal. Many people have high bills cause they leave the heater plugged in too long, Those heater draw a lot of electric power, when plugged in, even with their auto shut off. Keep a good pump running and big filter, use heater when needed and all should be well.
It is not that I cannot change or am not willing to change Mitch, it is only that I know all too well of how my pond operates best, and am not willing to risk the life of my very old koi on a 40 watt pond breather.