When RAID is working, it works great. . . When something goes wrong with RAID, it's quite often catastrophic. I've seen more than my fair share of RAID issues over the years.
Even if I wanted to use RAID (which I don't), I can't b/c the current DNS firmware doesn't fully support seagate 1.5 TB drives (yet). Short of cobbling together a NAS using an old box and FreeNAS, I was looking for a quick and dirty solution to backup ~2.5 TB of data rather quickly.
Regardless of RAID's maturity and stability, storing data on a single drive is always safer than striping information across multiple hdds (with multiple failure points). Not to mention the need to maintain spare hdds (ideally the same make, model, and lot number). Also, if the DNS fails (independent of the RAID array), the only way to access the data is to purchase an identical DNS. There are too many potential failure points. I will sleep better using third party software to perform manual backups and lose 50% of my storage capacity rather than using a RAID solution and risk (albeit a minimal risk) losing everything for that addition ~25% of storage.