This is actually kind of a cool feature. I often bring a 20*F sleeping bag on most summer trips, even though the nighttime low is only forecasted to be 45*F, and the temperature at the time I go to bed is more like 55*F. In this situation, the bag is too warm to zip up in the evening (I'll either wake up sweating, or I simply can't get to sleep if I'm too warm), but it's often drafty if I leave it totally unzipped. Sticking my feet out the bottom doesn't work because my feet get cold faster than the rest of my body. The best way to vent off the excess heat would be a vent over the core, like this. The only problem is that the bag is not designed for side sleepers - you can vent your front, but your back will still roast.
