Defragging will still help in this situation or any other raid solution.

The begining of that virtual drive is going to be physicially located on the begining of each drive, and (assuming all the drives are the same size) the end of the virtual drive will be on the ends of the physical drives. So, even though the drive the OS sees is not real, it still works like a real drive. It will take longer for the array controller to move the heads all over the drives to grab data instead of in a sequental pattern, just like normal drives.