Sometimes 6 or 60
For server-local storage, when the cost difference between a 6-disk RAID6 and an 8 disk RAID10 is just the two extra drives, I agree, RAID10 is a very good choice. As mentioned above, with large numbers of drives, there are substantial increases in cost in provisioning and running the additional enclosures to accomodate the extra drives required. (Imagine a 144 drive 12x12 RAID 60 - it would take a 240 drive RAID10 to match the capacity).
RAID 10 is still potentially vulnerable to a double-disk failure - if the second disk is the other half of the degraded subvolume. This isn't as unlikely as you might expect (that is, that assuming a second disk fails that it will be that drive), as the remaining disk in the set is under significantly higher load.
RAID 6/60 mitigate this by tolerating *any* second failure, and offer substantial increases in capacity and read performance owing to the wider stripe. Provided you keep each RAID6 set down to a relatively modest number of disks (depending on their performance/capacity ratio), recoveries can take an acceptably short time, and (with a suitably sized controller cache), the write overhead is modest.
