Well I have a few reasons.
1. We have multiple offices with up to 20 users VPN into the system as well as 20 users in our local office that access the shared resources very frequently. My main concern here is high bandwidth for multiple users and the ability to have multiple RAID arrays setup.
2. I also want to host a FTP on this second machine. I want the FTP physically separate on a different array then our shared files. My 12 disk box gives me the ability to have multiple arrays setup.
3. Scalability. We have massive amounts of archiving we do for our clients, with this setup when I run out of space I simply would add another 12 bay SAS box and daisy chain it off of the original.