Hello @stgraber Thank you for this reply. I’ve been using straight LXC (without LXD) for years, and all of my containers use simple ext4 filesystem directories, like so:
/var/lib/lxc/vps00/rootfs/...
/var/lib/lxc/vps01/rootfs/...
/var/lib/lxc/vps02/rootfs/...
etc.
Going forward I’ll use LXD, but I don’t need to clone or snapshot my containers. I manually do that once in a blue blue moon with cp -rp, to keep things simple and transparent. (I also plan on using static IP addressing, which I’d manually update after such a clone, as well).
So besides cloning and snapshotting, are there other consideration for using btrfs or zfs? Athough I’m an old hat with LXC, I’m brand new to LXD (the snap version by the way).
Remember, simple, transparent and not over-engineered is always my preference. 
Still, if there’s a good reason to adopt these backends, do tell.
Thank you @stgraber and anyone else who chimes in.