Well, what it would do is protect you against making mistakes. A RAID is going to give you, as you said, second-by-second duplication -- so, disk corruption (non-hardware) is going to be replicated across both, as will file deletion/overwrite/etc.
The SD! backup would avoid these things. But it obviously wouldn't recover from a single-volume hardware failure as quickly, because in a typical RAID you could just swap drives and have it rebuild without any downtime.
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--Dave Nanian
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