The main benefit it that buy using a top fan as well as a rear fan you're effectively doubleing the air extraction, which means you can use lower rpm fans and still get more air flow.
This end up with a quieter case.

The noise a pc make is the combination of the speed of the fans and the number of fans.

You can still flip the psu over in a case like this so the fan is on top, and thus drawing air from within the case (this is common on most of the Antec cases)
Although it's not going to help vent air from the cpu area.