I believe the side vents were intend for people that use engine ambient air (from air pushed through the rad and passed by engine) to heat their cab, when its warm out they have no choice but to get hot. They do look cool though.
We don't let any engine heat/air into the cab. Completely sealed off. All the heat comes only from the 2 heaters we have in the front, which we have total control of water flow and fans. We can make it as cold or as hot as we want it. This is superior in my opinion, because of the control over heat, and the less gas fumes that you have to smell that are associated with older motors.
