Neil's on the money, the amount of heat emmitted from the engine, gearbox and rear diff during racing conditions is astounding. Covering the whole underside of the car would creat serious heat issues. I'd say having a front diffuser/undertray would suffice, can't really be a bad thing could it??
And Beer Baron is right too, having a completely flat undertray is no good, I've seen a few diffusers on race cars and they all have the centre cavity. Why does it have to be there? NO IDEA!!!!
I'm sure Roy knows all the technical information on why it works, so c'mon Troy, enlighten us!