What many people fail to understand about honeybee colonies is that a great deal of their energy is spent simply maintaining the atmosphere inside their hive (or tree, or whatever cavity they are living in) within quite precise parameters of temperature and humidity.
A hollow tree, with its thick walls and many feet of natural insulation above and below, is relatively easy to manage, as the cavity is well insulated from the outside world and thus requires relatively small inputs from its occupants. A standard hive, however, has much thinner walls and little insulation above or below, and so requires a great deal of work to ensure that the brood nest can be kept at 35 C +/- 1 degree.
In a cool climate, which I would define as one in which the daytime air temperature rarely - if ever - exceeds brood nest temperature, then any time you open a hive, you allow heat to escape. If you use a typical thin-walled hive, heat is escaping all the time and causing the bees extra work replacing it. If you leave a hole open at the top of the hive, you create a chimney effect, which draws cool air in from below, causing bees even more work.
In a warmer climate, you can create similar problems for the bees by allowing too much over-heated outside air into the hive, and beekeepers in places like Georgia and Florida often report melt-downs when air in excess of 40 degrees C begins to soften wax and cause combs to collapse.
Good insulation and minimal ventilation helps the bees much more than gaping holes in their hives.