For the price of a hole cutter, why would you spend the time to heat a can, handle with oven mitt, risk screwing up the hole, because you can't exactly measure and risk having it not work as intended? Give me a $7 hole cutter and I can cut a hole in less time than it takes to heat the can. I can also cut holes in all sizes and not have to look for a soup can, a big can of beans, a small coffee can, a big coffee can, a 50 gallon drum...
It's also really tough to find a port hole protector in the right size. They come in 4, 5 or 6" and not Soup can, Bush's Baked Beans, Coffee can sizes!