Gotta be honest, I find the idea of a bootable backup disk to be defective. What if it was an OS bug that wiped out your volume, and booting the backup does the exact same thing to the backup, because of the same bug?
Well, having relied on a similar product (Carbon Copy Cloner) to keep a bootable backup in addition to time machine backups for myself and members of my family to reduce MTTR for many years, I find the idea of being unable to trivially recover from a hardware failure in a few minutes to be defective.
Any of our clones can be booted on any similar system. On a few occasions people have used borrowed machines until repairs were complete without relying on cloud or internet connectivity. Of course these are all older systems and working while booted from an external drive is definitely a degraded experience akin to driving on a spare tire until you can get to a proper service station.
I could keep going making a list of all the ways I’ve found a bootable clone of a MacOS machine to be useful, but damn, I’d hate to live without it.
I’ve been using normal Time Machine backups, together with iCloud (not really backup), occasional copies of data I care about to external SSDs, and occasional encrypted backups to remote storage (rsync.net)
But it never even occurred to me to want a bootable backup or include the OS in it. In fact whenever I upgrade my MacBook, I choose to set it up as a fresh device and don’t use any of the tooling to copy everything-and-anything.