Contaminants get in there anyway, whether the case is open or not. (Dust)
What really harms airflow is the lack of a vacuum created by the rear case fan and power supply. With the case closed, the vacuum literally sucks the heat out. With it open the heat has nowhere to go, and it stagnates.
This theory only works if you have good airflow in the first place however, if you have cables and wire ties and everything strewn everywhere it does not work as well.