this is about drives that are not plugged in. are you saying parity would let you simply detect that the data had gone bad? increasing the number of drives would increase the decay rate, more possibilities for a first one to expire. if your parity drive expired first, you would think you had errors when you didn't yet.

No, I'm talking about parity raid (raidz1/z2/z3, or, more familiarly, raid 5 and 6).

In a raidz1, you save one of the n drives' worth of space to store parity data. As long as you don't lose that same piece of data on more than one drive, you can reconstruct it when it's brought back online.

And, since the odds of losing the same piece of data on more than one drive is much lower than the odds of losing any piece of data at all, it's safer. Upping it to two drives worth of data, and you can even suffer a complete drive failure, in addition to sporadic data loss.