Scrubbing helps fix random errors in the drive data, but it won't prevent correlated disk failure. If a drive fails during the scrub, then you still have to replace the disk, and it's still very likely that another disk can fail in that time. If you just want to eliminate lemons, then you can do a burn-in test before adding it to your array, but IMO even bad disks usually don't fail that quickly.

Scrubbed as in erased? Where do you temporarily place 1PB+ of data while that’s happening?