I do understand the reduction in fault tolerance, but I still don't agree with the logic. Dual parity drives protect you from two random failures ( long shot) and only then if one of the drives happen to be the parity drive (longer shot). Two failures in the array loose data even with three parity drives. I'm not trying to pick a fight here.
I'm not trying to pick a fight either, but what you say isn't true. Multiple parity drives protect you from multiple drive failures, regardless of which drive it is. Frankly, I don't understand the logic that would suggest that a person's needs would only fall into one of two camps: either fault tolerance of one drive is sufficient, or you need fault tolerance for all your drives. Everyone has a different amount of data, and different needs. Does it not make sense that the appropriate number for someone could be somewhere in between "1" and "all"?
To be honest, I didn't realize the 2nd parity drive option in unRAID was just a mirror of the 1st parity drive. I agree, that is of little benefit, since that only provides additional protection for one specific circumstance. FlexRAID's multiple parity options allow for multiple drive failures up to the number of parity units you have, regardless of which drives fail.
It's simple statistics. The more drives you have, the greater your chances of something going wrong, and the more often it happens. The more parity drives you have, the more protection you have against Murphy's Law. I have 21 drives in my server, and two parity drives. And to be honest, if 2TB drives were still $60, I'd probably have a 3rd by now, and will be adding one when the prices come back down. It's cheap insurance. 1:1 backups are even safer, but would require 42 drives, which is crazy. Multiple parity drives allow you to scale your fault tolerance to whatever level you feel is appropriate for the number of drives you have, and your budget (and space constraints, ports, etc.)
I agree with most of this, and I have 2 parity drives maintained by SnapRAID, which works well for me.
But I cannot resist pointing out that the statement "1:1 backups are even safer" is not necessarily true. If you have the equivalent of a RAID 1 mirror, and you have a drive fail, then you are down to a single point of failure -- if your backup drive fails before you duplicate it, you have lost data. With dual- (or triple-) parity, after you lose a drive, you are not yet down to a single point of failure -- you can still lose a drive (or two, for triple parity) during the rebuild without losing data.
The mathematics are complicated by the fact that restoring with parity requires reading multiple drives, while restoring from a mirror typically requires reading only a single drive. But if you use the rule of thumb that the most robust solution is one that most often avoids degrading to a single point (i.e., single HDD) of failure away from losing data, then you will usually make the right choice.