The air duct is really more for P4's than AMD based systems.
P4's are known to run very hot. Most notable the prescotts. If you notice in the spec. on most sites, the case will say, "P4 Prescott Ready." or something like that.
The prescotts/HS&FANS cool much better when the hot air in the HS fins is pulled out of the case by the fan on the HS then through the airduct, sometimes with an additional fan and out of the case.
It's better then the HS/FAN then the system fan trying to remove the hot air... It also keeps the system temp down...