Indeed, remember the cMP only actually had one 'exhaust' fan...all the others were at the front or in the middle, and I believe the new Mac Pro does have an exhaust blower?
EDIT - as evidenced below which is the image I was looking for!
You also might be surprised to know that a lot (most?) rackmount servers don't use exhaust fans, or only have them in the PSU, they do however have a big array of fans at the front or in the middle betwixt drives and board and a decent airflow path to the rear, but exhaust fans? not so much.
You can suck air through, you can blow air through, you can even suck and blow, the important bit is that it's moving through...that's not to say they coudln't move more air through with a different setup but as long as it's adequate then that's what matters, otherwise you might as well just ramp all your fans to max and tape a vacuum cleaner to the back as well just to be sure ;-)