Not leaving stuff in cans is because after opening the contents are exposed to air. Exposure to air along with alreadt acidic contents are quite corrosive. That could cause unprotected cans to begin deterioration introducing the corrosion products to the food. Today most cans are coated and probably not nearly as subject to that potential problem. We also use those special can openers, but don't leave anything in the can for more than a day or two.