If you have an extraction fan and it is pumping air out of the tent, then obviously air from elsewhere is coming into the tent... (You can't create a vacuum if that is what you're worried about.) If your tent is sealed too tight (which I think is pretty hard to do) then the air will move more slowly... Just add a little hole on the opposite side of your tent (from the extraction fan) if you feel the air is not moving as fast as it should. (I believe your out-going fan should move an amount of air equal to the volume of your grow area in 4-5 minutes. Passive air intake should be enough for most people, unless you have a really large grow.
Also the fans DO cool the plants... just like with humans, fans help plants and humans release moisture/gases quicker... and when a plant is hot they need to get rid of a lot of gas and water vapor and the fans aid in that process.