A major kernel version update or an update to xorg could make one of my computers unbootable since I use nvidia-390xx drivers from the AUR. So I would have to wait before updating until the AUR package gets updated, but this could be a problem if I wanted to install new software (or if I simply forgot and just updated without thinking about it). So I think this could potentially be very useful to people like me. I could use weekly-deferred and in that week, and update to the AUR package is bound to occur, so I would just update everything at once.
It would, at least, remove one thing to think about. It would dull the bleeding edge just slightly, so to speak…