ZFS auto-heals on read so the files you touch regularly are fine. You do need to scrub regularly to prevent bit-rot across all your files. I go with once-a-week as well.
Only when the array is mostly empty. For several years hardware RAID controllers have been rebuilding only used space, too. Really, ZFS isn't that much of the miracle some want it to be.
but that's not what we were talking about! no-one was saying "zfs sucks as much as raid but at least it rebuilds faster afterwards". the implication was that zfs avoided the problem in the article (when, it seems, both zfs and raid need to be scrubbed, and both avoid the problem when that is done).