Druid, for your first quesiton (about why 2 drives and not 1), if you have ONE mid size hard drive (say 250gigs), and it gets filled up, it would be kind of silly to get a 500 Gig drive, move everything off the 250 drive and toss it.... right?
Also, I want to note here..... RAID 1 IS NOT A GOOD MEANS OF BACKUP.
Many people think this is the case, as I too once did. RAID 1 is simply to protect data from DRIVE FAILURE, that is ALL. RAID 1 does NOT protect against user error. If you delete it from the array, it's gone. After a LOOOOONG night and MANNNNNNY drinks at a pub, I used my computer, and woke up the next morning to find over 300 GIGS of music, photo's, and movies were deleted from my network drive... I gave up RAID 1 at that point. LUCKILY, I had some nice recovery software, and recovered 90% of my pics, 100% of my music, and 50% of my movies. The movies were no big deal, because they were ripped to the media server from my DVD collection.