Quick IDE 101:
There are 2 IDE channels for a reason. While one channel can support up to 2 devices, if the devices are transferring data between one another, they are forced to do so at the speed of the slowest of the two devices.
Example: Primary Master on IDE_1 is a UDMA 5 (100) harddrive. Primary Slave on IDE_1 is a UDMA 2 (33) dvd-rw. If you copy files from a CD or DVD, you can only do so at UDMA 2 (33) speeds... which pretty much kills the whole benefit of having a fast UDMA 5 (100) hard drive. The other big downer is that while one device reads or writes, the other has to sit and wait for it to finish. A bottle neck is created, which severely impacts read/write performance.
If both devices were set as Primary Masters, one on IDE_1 and the other on IDE_2, much of the bottleneck would be relieved due to the UDMA5 (100) device being able to burst data at a much higher rate. Both devices can both work at the same time, so they don't have to "take turns"... the data gets transferred on the fly. A similar situation occurs if the hard drive is on the SATA bus, which is why I will be most interested in snatching up a 2.5" SATA Hard Drive as soon as they are available... although I believe that the SATA functionality has probably been disabled on the Z71V. Still, it would be worth a try...