Why does RAID 1 "Mirroring not provide performance improvements"?
My guess would be that Apple prefers to have one drive handle only write processes as a backup and read/write from whichever one it considers the main drive. As to why this is, I have absolutely no clue. Seems kind of like a big oversight on their part. I've always kind of gotten the feeling that software RAID is pretty low on Apple's priority list.