If they pumped it in it would hardly be able to get back out and eventually it would stop everything from working - or when you switch it off after a certain amount of time it would give such a blow of air when the fan switches off that it would destroy the case!
pump air in in the front of the case and draw it out of the back of the case - this way you get good airflow through the case and everything stays relatively cool
yeah i have a 120mm in front now pumping air in and it goes straight through t back where theres 2x80mm fans to push hot air out! (I also got a fan in side window(right above the processor fan! and it sucks air in which appears just to get blasted into processor fan then))