Warning, the sound of a broken record coming up...
> An unreliable backup solution.
Nope. RAID is not a backup solution. It provides redundancy so the array can survive an event like a device failure (and so the data survives as a consequence) without significant downtime for repair (with zero downtime if you have hot-swap hardware) but it does not, and is not intended to, protect the data from the huge list of other things that can affect it.
RAID is redundancy for reliability purporses, not backup purposes.
Hot spares do not provide less downtime compared to a supported drive failure (ie. 2 disks on raid 6). they just reduce the mean time to repair. Handy if your disks are at a remote site or your dealing with very large arrays but likely a waste of spindles if your sitting next to your data all day.
Not hot spares (which you are right, don't reduce array repare downtime): hot swap (which can). If a hot spare gets used you still need to swap out the broken device (its replacement becoming a new hot spare).
Wether you are swapping out a replacement for an active drive or swapping out a dead drive with what will be a new hot spare (a drive that was previously the spare now being an active drive in teh array) whether you have hot-swap or not makes a difference. Without, you have to power down the array for a short time which in an environment that needs availability to be as high as possible could be an issue.
> An unreliable backup solution.
Nope. RAID is not a backup solution. It provides redundancy so the array can survive an event like a device failure (and so the data survives as a consequence) without significant downtime for repair (with zero downtime if you have hot-swap hardware) but it does not, and is not intended to, protect the data from the huge list of other things that can affect it.
RAID is redundancy for reliability purporses, not backup purposes.