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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I do see it definately being related to the air\AC\Heater because, and correct me if I am wrong here, in order for the air to flow to the back of the vehicle there needs to be some suction pulling it. Removing the vent essentially leaves no way for the air to circulate to the back of the vehicle. And since there is no rear vents that i can see.....this would mean that the rear passengers and cargo area wouldn't get air circulated to them nearly as efficiently. </div></BLOCKQUOTE>

90 Percent correct. It does help to relieve pressure inside the vehicle when the door is closed or temps rise.
The volume of air that is dispersed inside the cabin when a door is closed is considerably greater than the size of the valve. Its primary purpose is for airflow for the HVAC system. That might be the reason that the person responsible for this vent in the product engineering area of GM is also the HVAC product person.