Well, this article disputes what I've said pretty well:

http://woodheat.org/the-outdoor-air-myth-exposed.html

I have some doubts about their conclusions and concerns based on the implementations that they studied, and especially as they pertain to tents, but after considering those doubts I have to conclude that it may not make a spits worth of difference either way as far as heat efficiency goes. However, the outside intake might be of value if there ever were an issue with wind and reverse pressurization.